Erik ten Hag admitted Manchester United had been taken to school by Manchester City when they were thrashed in October, but he delivered on his insistence that lessons had been learned at a tumultuous Old Trafford on Saturday lunchtime.
Arsenal, Liverpool and Tottenham have all been beaten here by United this season but victory over Premier League champions City, and the manner in which it was achieved, is by far the most powerful statement of Ten Hag's tenure.
Manchester City will believe this derby turned on a controversial decision that ruled Marcus Rashford was not interfering with play when Bruno Fernandes equalised with 12 minutes left.
In the ensuing chaotic few minutes, Rashford swooped for a winner to give United a 2-1 win.
The subsequent celebrations at the final whistle carried the conviction that this giant club, which lost its way so badly, may just be finding itself again under Ten Hag.
Marcus Rashford has scored in each of his last nine appearances at Old TraffordUnited have now lost only one of 19 games since that 6-3 humiliation at Etihad Stadium, giving weight to Ten Hag's belief a harrowing experience would be used as a force for good.
The conclusion to this derby may have been highly contentious, and there had to be sympathy for City's complaints, but United's performance, character and unity justified the growing optimism around Old Trafford that has replaced the despair of the season's early days when they lost at home to Brighton and were then humiliated at Brentford.
Casemiro, to use Ten Hag's words, has put cement between the bricks since his arrival from Real Madrid but so many other factors are coming together as United now stand only one point behind City - a state of affairs that would have had anyone suggesting it laughed out of town a few months ago.
At the heart of it all is the tactically astute, disciplinarian Ten Hag, who is putting an even stronger stamp on Manchester United with every passing week, looking more at home and suited to the task than many who have preceded him in recent years.
Ten Hag has restored belief, instilled smart game management and also made it clear who is in charge when Cristiano Ronaldo was hustled off the premises sharpish following his insubordination.
Even in-form Rashford did not escape when he was dropped at Wolverhampton Wanderers after being late for a meeting.
Is there a greater beneficiary of Ten Hag's influence than Rashford?
The winning goal means Rashford has scored in each of his last seven games in all competitions, eight in total, and has been on target in each of his last nine appearances at Old Trafford, the first United player to do so since Teddy Sheringham in late 2000.
Rashford's joy was in sharp contrast to City's goal machine Erling Haaland, who had a rare fruitless day as he only touched the ball 19 times, with just five inside United's box.
It is far too early to get carried away by talking titles and measuring gaps being closed to Manchester City, who have dominated not just United but pretty much everyone else domestically in recent years.
What can be stated without contradiction is that United are, at last, moving back in the right direction. It can be seen on the pitch and witnessed in the stands at Old Trafford, which was in ecstatic mood at the final whistle.
Ten Hag still has much work to do after the barren years of under-achievement but to have United third in the Premier League and right on City's heels is an outcome everyone with any association with the club would have snatched at in August.
He is also making decisions that work, one here being Luke Shaw's deployment in central defence, a move that raised eyebrows given he was operating in the same areas as Haaland.
Shaw was outstanding, especially in stoppage time when the defender rose to win aerial duels on several occasions during a lengthy game of head tennis on the edge of the area.
Ten Hag's arrival, and his measured approach, meant this was always going to be a case of managerial evolution rather than revolution - but the best way to prove the graph is on an upward curve is to beat the champions, especially when those champions happen to be Manchester City.
