In his regular BBC Sport column, Robbie Savage looks at Manchester United's trip to Chelsea on Saturday and the battles that could decide whether Louis van Gaal's in-form team can stop the Blues taking another step towards the Premier League title.
Chelsea will be champions if they win their next three games but I think they will be happy just to avoid defeat against Manchester United on Saturday.
United's confidence is sky-high after their run of six straight wins and their performances have been as impressive as their results.
I have to admit I did not see this revival coming. When I picked my top four a few weeks ago, I thought United would miss out.
I predicted they would lose against Liverpool and Manchester City and draw with Tottenham, but they blew all three of them away.
Next up are the Premier League leaders. There will be some fascinating battles all over the Stamford Bridge pitch but, if United maintain their form, they will beat them too.
Settled side makes a difference for United
A lot has changed about United since they drew 1-1 with Jose Mourinho's side in October thanks to an injury-time equaliser from Robin van Persie.
United played a 4-2-3-1 formation that day and their back four was the eighth different centre-half combination that United boss Louis van Gaal had tried in his first nine league games.
He chopped and changed his team and formation a lot in his first few months in charge, sometimes by choice and sometimes because of injuries or suspension.
But we have seen a more settled United line-up in the last few weeks and they are benefiting at both ends of the pitch. They have clicked up front, and they have also been much improved at the back.
Earlier in the season, United looked disorganised in defence and relied heavily on goalkeeper David De Gea. That is no longer the case.
Antonio Valencia, Jonny Evans and Chris Smalling replaced Paddy McNair, Phil Jones and Luke Shaw, and Rojo moved from centre-half to left-back.
There have been changes since then, with Jones returning for the suspended Evans and Daley Blind taking Rojo's place when he got injured.
But there have been no more dramatic overhauls in terms of personnel or formation.
United's defence is still not perfect but since that Swansea defeat, they have limited the opposition's chances and De Gea has been called on far less.
I don't see that changing against Chelsea.
The Blues are not the same attacking force without the injured Diego Costa and, in any case, I think Jose Mourinho might take a cautious approach because a draw will do for him.
|Manchester United's improving defence|
|Premier League||Up to 21 Feb||After 21 Feb|
|Goals conceded per game||1||0.67|
|Shots faced per game||10.85||6.83|
|Shots on target faced per game||3.96||2|
|Errors leading to shots per game||0.81||0.33|
A lot has been made of how Van Gaal has stumbled across the 4-1-4-1 formation that is working so well for him at the moment - that Wayne Rooney is only back up front because of Van Persie's injury, for example.
But it is the United players who have made it work. In their last four games, they have shown how well that formation suits them.
At the moment the United team picks itself.
Carrick's key role in defence and attack
Whether United are unchanged might depend on whether Michael Carrick is fit after he was forced off before the end of the Manchester derby.
They need him to be - they have only lost one league game this season when he has started.
Carrick has an influence all over the pitch, and he is part of the reason Smalling has looked so good recently.
Without Carrick in the team, Smalling has to ping the ball forward and his passing, especially the weight of his pass to the front man or a midfielder, is poor.
It is a lot easier when he just has to find Carrick 10 yards away and let him be the one spraying the ball around. That way Smalling can concentrate on defending, and he has been brilliant at that side of things.
I am not sure Chelsea can stop Carrick, because good players like him will find space no matter what you do.
And the problem for Jose Mourinho is that Carrick's presence also helps United get the best out of Fellaini too.
Fellaini operates on the left of midfield for United and is the target for so many passes forward when they build from the back. Carrick is one of the players that likes to look for him.
I think the big Belgian is the best player at taking the ball down in the Premier League. Off his chest, or wherever he controls it, most of the time it sticks and he can bring others into play.
United have been given some stick for playing long balls up to Fellaini, but their play is far more measured than just hoofing it and hoping for the best.
Fellaini will often drift wide and sit on the opposition right-back because he knows he will almost always win a high ball against him.
That was the tactic that worked for United's equaliser against City on Sunday - Fellaini beat Pablo Zabaleta, allowing Ander Herrera to cross for Ashley Young to score.
It might not be so easy against Chelsea because their right-back Branislav Ivanovic is also strong in the air.
But Fellaini will try to get a chance against Chelsea's left-back too. He always goes to the far post whenever a cross is coming in, even from the left, and whether he is up against Cesar Azpilicueta or Filipe Luis, they might need some help.
He did it against City for his goal, when you could see him wait beyond Gael Clichy and call for the ball because he knew Clichy could not stop him if the delivery was right.
Fellaini will have it a lot tougher this weekend than he did against City, when he completely bossed Yaya Toure in midfield.
John Terry and Gary Cahill will be watching his runs into the box and Nemanja Matic will track him outside the area but the fact remains that, if Fellaini gets the right service, he is almost unplayable.
Mata vs Hazard could be the decisive battle
Fellaini is not the only player that Chelsea have to watch. Juan Mata goes back to Stamford Bridge this weekend for the first time since he left to join United for £37m in January 2014. He is in great form and he will have a point to prove.
He was adored by Chelsea fans and was brilliant in the number 10 role, but Mourinho got rid of him because he didn't have the work-rate he demands of his forward players.
So Mourinho might be surprised by how well Mata has performed since he came back into the United team on the right of midfield in their win over Tottenham. I know I did not expect it.
He has not tracked back in every game, but then he has not had to - against Spurs and Aston Villa when United dominated possession he did almost all of his work in the opposition half.
|Juan Mata's touches in his last four games|
|Game||Touches in own half||Touches in opp half|
|Vs Aston Villa||15||89|
|Vs Man City||27||30|
But against City and, notably, Liverpool he did far more, and showed he has found a balance between defence and attack.
His duel with Alberto Moreno in United's win at Anfield might give us a clue of how he will cope with Eden Hazard running at him down his flank.
|Juan Mata's touches vs Liverpool|
Mata got the better of Moreno in every way, losing him for both of the goals he scored that day and keeping him quiet at the other end too.
Hazard will be a much more difficult opponent, in both directions, but the kind of form that Mata is in could mean he is the match-winner at Stamford Bridge yet again.
Robbie Savage was speaking to BBC Sport's Chris Bevan