Darren Fletcher joins Alan Shearer, Phil Neville, Jimmy Floyd Hasselbaink and Gary Lineker at Stamford Bridge on Monday for live coverage of Chelsea v Manchester United in the FA Cup on BBC One and the BBC Sport website from 19:00 GMT.
Manchester United manager Ole Gunnar Solskjaer's tactical flexibility has already seen his side win at Tottenham and Arsenal in the past month and it could be the key to them beating Chelsea on Monday too.
United will be without Jesse Lingard and Anthony Martial for their fifth-round FA Cup tie because of injury, but Solskjaer will still have plenty of options over how he sets his team up, and crucially he will be able to change things during the game too.
That flexibility is the biggest difference between United and Chelsea at the moment, because we know what we are going to get from Maurizio Sarri's side.
The Blues boss is not going to move away from his 4-3-3 formation, and his players are very restricted in terms of alternating positions and where they operate on the pitch.
That can be a strength as players know the system so well, but at the moment being so rigid is also a weakness, especially in a scenario like this one.
Chelsea have suffered some hefty defeats in recent weeks, including a 4-0 loss at Bournemouth on 30 January. Sarri has changed his personnel in midfield and attack since then - with Pedro and Ross Barkley coming in for Willian and Kovacic for the 6-0 defeat at Manchester City on 10 February - but his formation has stayed the same
It means playing Chelsea is an easier game to prepare for, because there are not going to be any surprises. United can look at their previous matches to identify any weaknesses and come up with a game plan.
But if that plan doesn't work for them, in an instant United can adjust their shape - and their players are adaptable enough to do that constantly if they have to.
In my time at United, Stamford Bridge was a notoriously hard place to go. They have only won twice there since 2002 and have lost 14 out of their past 22 visits.
But despite that poor record, I think Solskjaer will look at this game and think there are opportunities to exploit in the same way United did at Wembley and Emirates Stadium.
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Against Spurs and Arsenal, United essentially played with a midfield diamond and two forwards in wide areas. It worked very well.
I would not be surprised if Solskjaer did that again and used Marcus Rashford and Romelu Lukaku or Alexis Sanchez in those roles up front.
Behind them is where Solskjaer has got the flexibility I am talking about.
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With Paul Pogba, Ander Herrera and Nemanja Matic in the team, he has the choice of a single or double pivot at every stage of the game.
If they decide to match up with Chelsea and go with a 4-3-3, that could see Matic sitting with Herrera and Pogba either side of him, but they could easily switch to have Matic and Herrera protecting the back four together, with Pogba pushed forward as the number 10.
United played a 4-3-3 against PSG on Tuesday, with Martial and Lingard in wide roles. Lukaku, Sanchez and Rashford could make up the front-line against Chelsea if Solskjaer matches up against the Blues' formation and goes with Pogba, Matic and Herrera across his midfield
In contrast, 'Sarri-ball' will see Chelsea play with a single pivot in their 4-3-3, with Jorginho as that deep midfielder, with N'Golo Kante and probably Mateo Kovacic either side.
If you look at the physicality of all of those players, United's midfielders are a lot bigger and more powerful, which is another reason I think this is an area in which they will be able to impose themselves on the game.
Personally, I would put Pogba directly up against Jorginho - firstly to stop him, because if you give the Italy midfielder time he dictates play and makes Chelsea tick, but also because of the damage the France midfielder could do when United win the ball back.
Jorginho is not in that role to be a defensive midfielder - he is there as a playmaker. Pogba is much stronger than him and, in the form he is in, he will cause him all sorts of problems if they go to head-to-head when United attack.
There is more than one way for Solskjaer to do that, though. If he goes with the diamond again, he could put Pogba at the tip of it and bring in another midfielder - say, Fred, to play on the left.
That way, United would outnumber Chelsea four to three in midfield and still have Pogba sitting on Jorginho, who will be at the base of Chelsea's midfield no matter what.
Rashford and Martial played in front of Lingard in United’s 1-0 win over Tottenham on 13 January, while Lukaku and Sanchez played in those roles when they beat the Gunners 3-1 at Emirates Stadium in the FA Cup fourth round on 25 JanuaryFlexibility would suit Kante
Paris St-Germain did a superb job on United's midfield last week, playing five in the middle with Marquinhos tracking Pogba to stifle him completely.
Kante is the only Chelsea player who might try to do the same thing on Monday, but I can't see it happening.
Even if he does follow Pogba, the former Juventus player can just drop on to Jorginho and if Kante is in the same area too that would leave a massive space elsewhere in Chelsea's midfield for someone else to exploit.
When 'Sarri-ball' is questioned, it is usually his use of Kante in a more advanced role that is brought up, but we have to remember he is not and has never been used in the Makelele role while in the Premier League.
Sarri also wants something different from his holding midfielder.
Jorginho is there to play the Andrea Pirlo role or, as it is known in Italy, the 'Regista'. He is not there to tackle and break play up, he is there as a deep-lying playmaker.
Even if the Makelele role was what Sarri wanted, that is not what Kante has done in any of the other Premier League teams he has been in.
I am sure he could do it, but when he was winning the title with Leicester and again under Antonio Conte at Chelsea, Kante was not the midfielder who sits in front of the back four on his own.
Kante, 27, won the Premier League in his first season with Chelsea in 2016–17. He signed a new five-year contract with the club in November 2018
At Leicester, Danny Drinkwater went deeper to get the ball and Kante was the energy around him.
It was the same when Kante first moved to Chelsea under Conte and Matic was still there. Again, he was the more mobile of the two and Matic was the sitter.
One of the reasons Kante was so effective in those teams was actually because they had the same flexibility United do now. Sometimes he would find himself doing that sitting job because his position dove-tailed with what his team-mates were doing.
Now it is different because Jorginho has to be in his set position all the time so Kante's role involves dragging people away. He has to wait for the next phase of the move to see the ball, and before that he has to play further forward to create space for Jorginho.
That works when Chelsea are attacking but it does affect Kante's defensive contribution whenever a move breaks down.
As we know, he is a fantastic athlete but from up the pitch he cannot recover the bigger distances that are left when they lose the ball, and that leaves Jorginho exposed.
Herrera has a role to stop Hazard too
For United, getting things right does not just mean finding the right combination in midfield that allows them to dominate Chelsea. They have to be set up properly from a defensive point of view too.
Chelsea might be having a bad time of it recently but they still have some extremely good players, and they are very dangerous when they get it right.
We know Eden Hazard is a match winner, whatever else is happening on the pitch, so it is vital United's plan takes him into consideration at all times.
To do that, whatever United's formation is, I think Herrera has to play on the right side of midfield and he will have a dual role there.
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As well as being in midfield, part of his job will also be to double up with United's right-back Ashley Young against Hazard on that flank.
Hazard loves to come inside on to his right foot, which is when he is most dangerous.
Young will always try to show him down the line but you cannot leave your full-back one on one with Hazard because he is just too good - too sharp and too fast.
He can still get where he wants to go with a drop of the shoulder, so Herrera will have to be there to plug that gap every time.
Sarri needs time, like Pep didI want to play my football at Chelsea - Sarri
Sarri is getting some stick at the moment because of Chelsea's recent results, but his methods were never going to see an instant impact.
The easiest way to implement a different style of football and get results quickly is to adopt a defensive approach.
Conte, who won the title in his first season at Chelsea, is a good example of that. He was not worried about dominating possession and he made his team hard to break down and played on the counter-attack.
The harder way, as Sarri is finding at the moment, is when you are trying to keep the ball and be open and expansive.
Not only do you have to be good defensively, you have to try to break the opposition down and it is a much longer process to get right.
What is happening now at Chelsea is quite similar to what Pep Guardiola experienced in his first season as Manchester City boss in 2016-17.
Until Sarri nails down exactly what every player does, and they get that transition from attack to defence sorted so they know what happens when they lose the ball, they are going to continue to look vulnerable.
It might be that he does tweak the way he uses Kante and Jorginho because, like Pep, he realises some of the things that worked very well in a different league do not work the same way here.
It took Pep time to get it right, and a couple of transfer windows too, but look at where City are now.
If Chelsea believe in Sarri in the same way, which they must have done when they gave him the job, then they have to be patient too.
Darren Fletcher was speaking to BBC Sport's Chris Bevan.
