Jamie Redknapp picks out two players who could dent City's title hopes on Tuesday....
The Manchester derby and the next two Champions League games, for me, will shape David Moyes' future at the club. If he loses to City the pressure will be back on and people will say he can't win games against the best teams. But if he wins and can get a big result in the Champions League then everyone will say 'here we go'.
Until that moment comes, however, the jury is still out.
Ryan Giggs and Danny Welbeck were the two significant introductions. Some will say 'well done to the manager for bringing them in', others might say 'why has he not been playing them earlier on?'
Whatever your view, Welbeck was an ingredient United have been missing in the last few weeks - and a vital ingredient that all Man United teams I've ever played against have had - and that's pace, certainly in wide areas.
Before Wednesday everyone wanted the ball to feet, no one wanted to make the pitch big, no one wanted to stretch the play - and that's what Welbeck did from minute one the other night. He was willing to run without the ball and you could see he put that little bit of fear in the Olympiakos defence.
Giggs, on the other hand, was simply amazing. I've got to be honest, I couldn't believe what I saw from him on Wednesday. To be able to do what he did at 40 years of age was incredible - and you'll never see a performance like that from someone his age again.
To be so calm and so assured in such a vital game, a pressure situation, was very impressive. If you go into a game when you're 40 and you do ok everyone goes 'wow'. But he was the best technical player on the pitch by far. Every pass he made, the ability to pick it up first time, second time, play one touch, make tackles, interceptions... it was remarkable.
United built on the positive mood created on Wednesday when they beat West Ham. But the real test of the club's revival will come against City.