Twice a year, on transfer deadline day, English football is gripped by an incomprehensible mania but, even in that context, what happened at Stamford Bridge on 31 January 2011 stands out. Signing Fernando Torres for £50m was bad enough but letting Nemanja Matic leave as a makeweight in the deal that brought David Luiz to Chelsea now looks just as bad. Matic completed a £20.75m return to Stamford Bridge on Wednesday, which is an awful lot to pay for three years at finishing school.

Chelsea, perhaps, deserve some sort of credit for swallowing their pride and it may be they considered the alternative to be even worse. Manchester United were very interested in the player in the summer, and however galling it may be to have to pay £21m to recover a player who was once your own, it would be even worse to see him reinvigorating an ailing rival.

Matic, powerful and combative but good on the ball and capable of scoring goals, is exactly the dynamic midfielder United are lacking. Chelsea's immediate need for him is less obvious but it does perhaps indicate the direction in which José Mourinho is taking the squad.

The decisive moment for the Chelsea manager seemed to be the 2-1 defeat to Sunderland in the Capital One Cup quarter-final in December. "We may have to take a step back in order to be more consistent at the back," he said. "It's something I don't want to do, to play more counterattacking, but I'm giving it serious thought. If I want to win 1-0, I think I can, as I think it's one of the easiest things in football. It's not so difficult, as you don't give players the chance to express themselves."

The suggestion was he would switch from the 4-2-3-1 he inherited from Rafa Benítez – and that he himself played at Internazionale and Real Madrid – and return to the 4-3-3 he had deployed so successfully in his first spell at the club.

Sure enough, for the next game, away to Arsenal, he did just that, closing the game down with Mikel John Obi, Frank Lampard and Ramires in the middle, ensuring a stalemate. Since then the 4-2-3-1 has returned but so too has a steeliness that had dissipated in the autumn: in Chelsea's past six games in all competitions, they have conceded once, as though that quick re-immersion in 4-3-3 served as a reminder of defensive responsibility. Whether that means Mourinho sees 4-2-3-1 as the future is less clear.

After all, this is a squad that has seemingly been designed – in as much as anything at Chelsea is designed – to play 4-2-3-1. There is an abundance of attacking midfielders – Eden Hazard, Oscar, Willian, Juan Mata, André Schürrle and Kevin De Bruyne (plus Victor Moses, on loan at Liverpool) – and few who can play at the back of midfield – Frank Lampard, Ramires, Mikel and Marco van Ginkel (plus David Luiz, although Mourinho has made clear he does not see him long-term as a midfielder). Playing a 4-2-3-1 means perming three from six creators and two from four deeper players, or three in Van Ginkel's absence, which is a lot healthier than perming two from six and three from four as would be the case with a 4-3-3.

By bringing in Matic and selling De Bruyne to Wolfsburg, though, Mourinho changes that balance, and at least gives himself more options if he wants to play 4-3-3 more regularly – or, if he wants to carry on with 4-2-3-1 but use Ramires high on the right to check a marauding full-back, something Benítez did fairly frequently but which he has done only in the away defeat to Basel in the Champions League.

It is easy to see why Matic appeals to Mourinho. At a touch over 6ft 4in, he is physically robust, and offers both a goal threat from set pieces and extra security in defending them – an area in which Chelsea were surprisingly deficient towards the end of last year. He is also mentally resilient. This, after all, is a player who was rejected repeatedly before finally finding regular first-team football at Benfica.

Crvena Zvezda and Partizan Belgrade turned him down, so he was still playing third-flight football in Serbia with Jedinstvo Ub when he moved to the Slovakian club Kosice as a 19-year-old. He was fortunate there to be seen by Vladimir Vermezovic, a Serb coach who was working with Spartak Trnava.

He recommended Matic to Serbia's Under-21 coach, Slobodan Krcmarevic and it was after impressing at the European 2009 Under-21 Championship in Sweden that he moved to Chelsea. He made only two appearances there – and 27 on loan at Vitesse Arnhem – though, before being off-loaded to Benfica.

Even there it took some time to become a regular, but after Javi García had moved to Manchester City he blossomed, with Jorge Jesus, then Benfica's coach, insisting he was a better in the holding midfield role than the Spaniard. The proof of that came in July when Matic was named Portugal's player of the season.

At Benfica, Matic tended to play either as the lone holder in a 4-1-3-2 or as one of a pair of holders in a 4-2-3-1. In that sense he looks like direct competition for Mikel, the great survivor who seems to be rated by hardly anybody around Chelsea apart from Mourinho.

It would be misleading to say Matic is a more complete player than Mikel, who has shown for Nigeria that he can play further forward, but it is certainly true to say that if he played alongside him, Matic would probably be the holder who stepped forward, and there is no reason why he could not play to the left of a central three.

Given he is left-footed, in fact, Matic may end up being Lampard's long-term replacement, although he will never offer the same late runs into the box or goalscoring record. He is such an obvious fit for this Chelsea midfield that it remains mystifying that he never got his chance first time round.

However embarrassing it may appear, it could be that £21m is not too great a price to correct the error of that day in January 2011.