Antonio Conte has earned a lot of praise since he arrived in London last summer but it will all count for nothing if he cannot hold his team together during the run-in.
He has a problem now, there is no doubt about that. Jose Mourinho, almost by accident, exposed what has been a very solid Chelsea defence to Marcus Rashford and Jesse Lindgaard's raw pace and they didn't like it.
I had to laugh at Mourinho taking retrospective credit for leaving Zlatan Ibrahimovic on the bench so that he could put out his "best team".
Shortly before the kick-off when he was asked about Ibrahimovic, Mourinho said he was tired. He has some neck.
To be fair, he stuck it out when he selected the team and I fancy that Mourinho has stumbled on a truth. Ibrahimovic has been fantastic all season but to use him effectively, Mourinho must allow him to drop into midfield every so often to look for the ball.
Naturally enough, this slows the game down and I believe that this limits Rashford's effectiveness. He has been picked in a variety of positions but not often enough up front.
Given the freedom to run with the ball and a different dynamic to Manchester United's play, he was able to do his stuff. I have a lot of time for Rashford and none at all for Mourinho's public criticism of the lad.
He would be far better off dealing with any issues he has with Rashford's lifestyle in private and on the pitch, pairing him with Ibrahimovic up front on a permanent basis.
Certainly, Mourinho cannot ignore the fact that his team played as well against Chelsea as Manchester United have since Alex Ferguson's time and that this happened with Ibrahimovic sitting on the bench.
I was disappointed with Conte's team and particularly irritated with Diego Costa, who seems to contribute less and less with each game I see him.
I trace this back to Christmas and the stories which surrounded Costa about a huge move to China.
I have no doubt that Costa's head was turned and that Conte and Chelsea had to do some very quick talking to convince him to hang around.
But once a player gets a glimpse of something else which is very attractive financially, his head cannot be in two places so his form suffers.
He couldn't stop scoring in the first half of the season. He was banging them in for Spain and for Chelsea in all competitions, amounting to 17 before the year turned Since then, he has scored just five goals in 14 weeks and only one in the last month.
Against Manchester United, like many of his team-mates, he was well below par and that must worry Conte.
Chelsea looked tired, almost complacent and even N'Golo Kante, the best player in the Premier League so far this season, had a quiet enough day.
I have to say, my confidence in Conte's ability to see out the season has taken a bit of a knock with this result and if Costa continues to play in such a feckless way, Chelsea might struggle for goals.
That's certainly not what Conte needs right now.