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.

United romped to a 4-2 win against City in the Manchester derby last weekend

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.

How United's starting XI lined up in a 4-2-3-1 formation against Chelsea on 26 October...

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.

After their defeat at Swansea on 21 February, Van Gaal changed his entire back four around for the game against Sunderland a week later.

... and their 4-1-4-1 against Man City on 12 April. Smalling is the only outfield player in the same position in both matches

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 Games 26 6 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 started 16 Premier League matches this season, and United have won 12 of them

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's goal against City was his fifth in the Premier League this season

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.

Marouane Fellaini took up a far-post position to score United's second goal against City

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.

Mata scored 18 goals and made 27 assists in 82 Premier League games for Chelsea between August 2011 and January 2014

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 Tottenham 15 47 Vs Liverpool 28 65 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