As last summer's transfer window closed in August, I could not stop shaking my head in disbelief as the biggest story of the day unfolded.

David Luiz back to Chelsea? Seriously? Even though there was TV footage of him walking through St Pancras station after his Eurostar had arrived from Paris, I couldn't work out why this reunion was taking place.

For that reason I had doubts about whether Antonio Conte actually wanted him. Luiz, after all, is not a defender in the uncompromising mould of Leonardo Bonucci or Giorgio Chiellini. They had been two of his trusted lieutenants for Juventus and Italy and they were high on Conte's wanted list.

Chelsea had also pursued Napoli's Kalidou Koulibaly, so the fact they were scrambling to put together a cut-price deal to bring Luiz back to Stamford Bridge suggested to me it was a club signing rather than one driven by Conte.

When you look back at his last two managers during his first spell in London, Rafa Benitez moved him into central midfield as he did not trust him to operate in defence. Jose Mourinho had similar mis-givings and readily sold him to Paris Saint-Germain. Both are coaches in Conte's mould.

So, too, is Unai Emery, who had been appointed by PSG to target the Champions League after lifting the Europa League three times with Sevilla. Yet his first big decision was to sanction Luiz's sale at an £18million loss.

The feeling was that top managers around the world could not trust the Brazil international and I have no doubt on August 31 that Conte would have felt that way.

It looked to be a transfer loaded with risk and the feeling was enhanced after his first three games. Chelsea conceded seven goals in that sequence, lost two Premier League games to Liverpool and Arsenal and Luiz was booked in the 4-2 League Cup win at Leicester.

But then Conte got to work and came up with the system that is propelling Chelsea to the title and has turned Luiz into one of the Premier League's outstanding performers. A measure of his new-found discipline is that his brilliant goal at Liverpool was his first of the year.

Chelsea's 3-4-3 formation has revitalised Eden Hazard and allowed Victor Moses to finally establish himself in the team but, more than anything, it has given a responsibility to Luiz that has brought a maturity to his approach and his game.

In the past, you would have been classed as a sweeper if you were put in the middle of a three-man defence. It was the role for the ball-playing centre back, so you would say that role was ideal for Luiz. In the current game, however, the middle man has to be much more restrained.

Look at the figures. So far this season Cesar Azpilicueta and Gary Cahill have made far more passes, their totals of 1,420 and 1,145 dwarfing the 827 of Luiz. To give you an idea of how much time these players have on the ball, Azpilicueta averages 77.2 touches per game. Luiz is on only 57.4.

I was critical of Luiz in the past for being too hot-headed and emotional — recall how he cried during the national anthems at the 2014 World Cup in Brazil — but Conte so far has stripped that from him.

It might be one of the manager's best achievements so far, curbing his natural urge to charge forward and improving his concentration to such an extent that those rushes of blood that led to aberrations have been eradicated.

Luiz sees things more clearly now. Last season I regarded Toby Alderweireld and Jan Vertonghen as the best central defenders in the Premier League but such has been Chelsea's defensive improvement — both teams have conceded only 16 goals — you have to put Luiz right up there.

He has helped keep 12 clean sheets so far but his best figure is the zero in the column reserved for errors that have led to goals. That is where defenders will always be judged and, so far, Luiz has been almost faultless, with Opta's stats showing just one error in 19 appearances that led to a shot.

Faultless is not a word I ever expected to use about him.

The first time I came across Luiz was when he played for Benfica. It was a Europa League quarter-final in April 2010 but the standout players in that team were Angel di Maria and Oscar Cardozo, the Paraguayan striker. Ramires also played for them, as did the Argentine Pablo Aimar.

We had heard the talk that Luiz was going to get a move to a top club one day but he played left back at Anfield and we won comfortably 4-1. It was a sign that there was plenty of improvement still to be made.

Nearly a year later, he made his Chelsea debut as substitute against Liverpool but it went almost unnoticed as it was in the game that saw Fernando Torres face us after his £50m transfer on deadline day. Luiz's appearance, more than anything, made him stand out in those games.

So as the transfer window closed this week, I found myself looking at Luiz and contemplating just how much has changed since he returned to the Premier League. The only reason I shook my head on Tuesday was because of his quality.

His strike for the free-kick, of course, was outstanding but so, too, was his all-round game and it left me thinking about the end of the season.

In the next couple of months, you see, all footballers will receive the voting forms for the PFA awards.

We already have an idea of what the shortlist will look like. It will be dominated by Chelsea players with Hazard, who is operating at his dazzling levels of two years ago, Diego Costa and the relentlessly impressive N'Golo Kante expected to be among the favourites.

Frank Lampard stated his case last week for why Azpilicueta should be a candidate, explaining that he is 'perfect' to play in that position on the right side of defence and does 'two jobs at once'. He has been a model of consistency.

Yet why should Luiz not be considered? He has always looked every inch the modern defender, a big, strong powerhouse who can move. He has always had talent but now he has added discipline it is all coming together. He will face Arsenal on Saturday in the form of his life. For that he can thank Conte.