Manchester United have won two of their last seven Premier League gamesIncoming Manchester United manager Erik ten Hag must perform an "open heart operation" rather than fixing "minor cosmetic things", says interim boss Ralf Rangnick.
Rangnick said it is "vital" they sign the "best possible players" to change the "attitude and energy".
"We need positive energy for the new team and the new manager," he said.
"If he gets that, if the right lessons are being learned, this could be one of the few good things about the season we are going through.
"At least you know which kind of screws we need to turn, things we need to change, and, if that happens, why should Erik ten Hag not be able to be as successful as he was at Ajax?
"It's not the manager's quality - it's changing all the other things that have brought the club into a position we are in right now.
"The good thing - one of the few - is it's crystal clear. You don't even need glasses to see the problems.
"Now it's only about how you can solve them. Not minor cosmetic things. This is an open heart operation. If everyone realises this has to happen and works together, it doesn't need to take years.
"It can happen within one year. Other clubs have shown it's possible within three transfer windows."
How Ten Hag made Ajax Champions League contenders'A lack of Burnley basics'
Rangnick is set to move into a consultancy role at United when Ten Hag takes charge.
Rangnick took interim charge in November after Ole Gunnar Solskjaer was sacked, but two wins in their past seven league fixtures have left United sixth in the Premier League - three points behind fourth-placed Arsenal having played one game more.
On Tuesday United were thrashed 4-0 by rivals Liverpool, who won 5-0 at Old Trafford earlier this season.
United have only once suffered a combined heavier defeat in a league season - 11-0 against Sunderland in 1892-93.
"In training we haven't been on the level I'd like," said Rangnick. "We are very far away from football I would like to play.
"Even if I watch Burnley and they are fighting to stay in league, the way they defended and didn't allow Southampton a single chance...
"This is what I mean by basics, and those are things we need to show in the next games."
Asked if Ten Hag will require patience, Rangnick said: "I would rather say the level of expectations. This has also got to do with patience, yes, but in a club like Manchester United it is difficult to tell people to be patient. It will take time.
"We have to face current situation. We lost on aggregate to Liverpool 9-0, Manchester City 6-1, lost to Watford 4-1 and lost to Everton, who are not full of confidence.
"This is just the sad reality. It makes sense to have sense of reality and not accept but acknowledge it. It would also help Erik ten Hag for the future."
