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A lot of people questioned Chelsea's decision to re-sign Brazil defender David Luiz from Paris St-Germain on transfer deadline day, but he is proving to be the perfect fit for their 3-4-3 system.

I understand why there were doubts - while Blues manager Antonio Conte is known for his teams' defensive discipline, Luiz's problem is that sometimes he will switch off and he can be a little bit complacent.

At 29, he has not changed and he is still that same player who can make mistakes, but the formation Conte has used during their eight-game winning run allows him to play to his strengths because of the players he has got around him.

Luiz played alongside Cahill and Azpilicueta and behind Kante and Matic in the first seven games of Chelsea's eight-game winning run. The 11 players above started six of those games - the only others to feature are Willian, who played instead of Pedro against Hull and Fabregas, who replaced Matic against Manchester City

In front of him, Chelsea's defence is protected by N'Golo Kante and Nemanja Matic, but the key for Luiz is that when Chelsea play three at the back, he does not have to mark anybody.

Gary Cahill and Cesar Azpilicueta do that, and do the dirty work for him.

Maturing, or enjoying his free role? Luiz made an average of 1.4 fouls per game in 2013-14, his final season with the Blues before joining PSG. That has dropped to 0.6 this season

When Luiz plays in a four-man defence and he needs to mark, that can be a problem for him. In this 3-4-3 system, however, he is more of a sweeper and makes fewer challenges than Chelsea's other defenders.

That suits him more, firstly because he rarely finds himself in trouble so is not in danger of making a mistake, and also because he is the player with the most freedom in that Chelsea team.

Chelsea defenders - tackles per game in the PL 2016-17 Before 1 October Player Since 1 October 2 David Luiz 0.9 0.2 Gary Cahill 2.3 1.6 Cesar Azpilicueta 3.6 *Chelsea switched to a 3-4-3 formation midway through their 3-0 defeat to Arsenal on 24 September but the first game they started using that system was their 2-0 win at Hull on 1 October

Luiz can bring the ball out or play it forward and he uses it very well, which is so important when Chelsea look to break quickly.

Playing him alongside Cahill and Azpilicueta means they have the ideal combination at the back.

'It's not just John Terry who is waiting for his chance'

Terry (back, right) started Chelsea's first four Premier League games of the season but has only played six minutes in the league since, coming off the bench late on against Everton. That is the only action he has seen during Chelsea's eight-game winning run, and he has been an unused substitute four times in those matches

Captain John Terry cannot get into the Chelsea side because of how well they are playing, particularly defensively. He has not made a fuss about it, however.

I am not surprised by that. As well as the fact the team are playing well and winning without him, Terry sees himself as a worthy captain and also a team player. He is part of the squad, and I think he understands the situation.

Of course he would like to be involved - he is 36 and at his age will know he does not have much of his career left - but he is handling it the right way and he still has a part to play.

There will be injuries and suspensions and moments when Chelsea need him in their title bid. At the moment he has done well just with the way he has handled the situation.

'Football is not fair, football is all about winning'

Chelsea have made fewer changes to their line-up than any other top-flight team in their 14 Premier League games so far this season. Four of those changes came in the six games before Conte switched to a 3-4-3 system. He made two changes for the first game they won, against Hull, and two more in the seven games since

Terry is not the only Chelsea player who is having to wait for a chance. Conte did not change his line-up very often even before he altered his formation to play 3-4-3 at the start of this winning streak.

Players like to have consistency because then they know what they have to do. If you change too much then you get confusion.

Of course what helps Conte here is that Chelsea are not in Europe. Compare the Italian to manager Pep Guardiola at Manchester City, who is at the other end of the scale.

City manager Pep Guardiola has made 46 changes to his starting line-up for Premier League games this season, the most in the top flight. Swansea have the next highest number, with 34

The Spaniard is too much of a tactical wizard, and he does it all the time. He keeps changing his system between three and four at the back and he is the kind of manager who puts players in three different positions in three different matches, which is too much.

Eventually he will find a system and things will settle down at City, but it is not working at the moment, especially in their defence.

I understand why Guardiola is frustrated because City create so many chances in all their games, not just in last weekend's defeat against Chelsea, and it looks unfair when they don't win.

But football is not fair, football is all about winning - however you do it.

'Chelsea doing things the Italian way'

Chelsea wing-back Victor Moses operated mainly in the opposition half when his side won 1-0 at Middlesbrough on 20 November, making 61 of his 74 touches in there as the Blues enjoyed 58% of possession. Against Manchester City, in their next away game, only 22 of Moses' 55 touches came in the City half. Chelsea had 39% of possession in their 3-1 win

One of the good things about the run Chelsea are on is that they are doing things the Italian way, by conceding hardly any goals.

That's not down to luck, because Conte's tactics are designed to put the defence first and make his team solid behind the ball.

Look at Victor Moses - he was a right-winger in the games where Chelsea dominated possession but against City he was playing as a right-back in and around his own area for much of the game.

He drops into the back-line whenever Chelsea don't have the ball which is another thing that helps Luiz - when you are one of five at the back there are not really many mistakes you can make.

Luiz joined Chelsea from Benfica for £21m in February 2011 and won the Champions League, Europa League and FA Cup before leaving for PSG for about £40m in June 2014. He returned to Stamford Bridge at the end of August for £34m

But when Chelsea do win the ball back they also have the right players to attack quickly and hurt teams. Eden Hazard, Diego Costa and Pedro are all clever enough to create their own space, and dangerous when they get it.

Put all of that together and you have a very good plan because you don't concede anything and are dangerous going forward too.

'Even on difficult days, Chelsea will always have a chance'

Chelsea confidence is up - Conte

In September, before Conte had switched to the 3-4-3 formation he used with Juventus and Italy, Chelsea took one point from three Premier League games.

He needed time, and I said that people should judge him after six months, not six weeks.

It turns out he has not even needed that long to demonstrate again what a good coach he is.

Conte has come to a different league and had to adapt to it like everyone else but, as soon as he could play his own system because he had the players to do it, then suddenly everything changed for him.

So I am not surprised by how well he is doing. He has a game plan that works and some very good players at Chelsea who he uses in the best way, and things have just clicked for them.

Can Chelsea carry on winning? I hope so, but there will come a moment when they get injuries or tiredness. Look at the next few weeks and the number of games everyone plays.

There will be days when they don't play well. When that happens the most important thing is not to lose and, with the way they are defensively, even on difficult days they will always have a chance.

Ruud Gullit was speaking to BBC Sport's Chris Bevan.