Unless the rest of his life is one enormous bluff, it is safe to assume Diego Costa is not a particularly successful poker player. It is never hard to know what he is thinking: he is an open book whether he is scowling in anger, scowling in frustration, scowling in irritation or, as has increasingly been the case recently, scowling cheerily.

He doesn’t just have a tremendously expressive face but also hugely expressive shoulders, and he seems to serve as a useful barometer of the team’s mood, a brawny canary in the coal mine of the Chelsea dressing room.

His face tallies with recent form: things are going well for Antonio Conte’s side and for him. He did not turn 28 until October but last season it seemed possible to imagine his best days were behind him. He seemed short of zest, like an ageing prizefighter, clinging desperately to his opponent and spoiling rather than actually fighting. This season, as he has rattled in 10 Premier League goals, Conte is, with some justification, describing him as one of the best strikers in the world. The outlook is good.

Costa scored 17 minutes into his Chelsea debut at Burnley on the opening weekend of the 2014-15 season and looked generally alert and aggressive in a 3-1 win. Chelsea had had two major deficiencies the previous season, a high‑class goalscorer and a midfield creator, and he and Cesc Fàbregas filled those two gaps.

From the moment Costa scored that goal there was an overwhelming sense that Chelsea would win the league, and they did, with the Spain international scoring 20 goals.

But the following season the problems were just as apparent as his excellence had been before. Costa didn’t look fit, which might have been down to his fragile hamstrings or a more general lack of enthusiasm.

He had clashed with José Mourinho after returning late from a trip to Spain. He became increasingly surly on the pitch, his diving and his attempts to wind up opponents more pronounced and so less effective – Gabriel Paulista falling for it when Chelsea beat Arsenal was the equivalent of an eager but dim labrador turning yet again to chase a ball its master hasn’t thrown.

Diego Costa squares up to Arsenal’s Gabriel, left, in last season’s Premier League match at Stamford Bridge as Petr Cech plays peacemaker. Photograph: BPI/Rex Shutterstock

There are countless incidents that could be highlighted as the moment when there was no coming back for Mourinho at Chelsea, but there was no doubting the significance when Costa tossed his bib away after the manager had made a third substitution without bringing him on in the game at Tottenham Hotspur. Background rumblings of discontent can be laughed off; something as publicly dismissive as that could not be.

Once Mourinho had gone, Costa refocused. Following four goals before Christmas came 12 after but the assumption still was that he would return to Spain. Perhaps at the start of the season that was still the case, but he has looked reinvigorated.

He looks leaner, lither and the suggestion is that he is keen to extend his contract and will begin talks in the new year. The old hunger is back and a focused Costa remains an exceptional centre-forward.

He began the season with four yellow cards in six games (and two near‑misses with reds) but that side of his game is now just an adjunct – perhaps a necessary one – to the centre‑forward play rather than being all he is notable for.

Significantly, he has not been booked since the 3-0 defeat at Arsenal; Chelsea’s six wins in a row without conceding have coincided with a run in which he has kept his nose clean. There’s a chicken-and-egg element to that, but Costa seems symptomatic of the new mood of purpose and confidence that has run though the club since that loss.

The suggestion from within the club is that for Costa the key game was the 3-0 win over Leicester, the second in the run of six. There was a point in the second half when he wanted to come off but Conte ignored him. The message was clear: Conte was the boss and Costa seems to have responded to that in a way he no longer did with Mourinho.

“He is showing his passion in the right way,” Conte said after Sunday’s win over Middlesbrough. “He is very focused on trying to help the team, with or without the ball.” There has been a general improvement in that area with Chelsea this season.

Whether it’s to do with the shape, Conte’s personality, or both, there has been a greater sense of integration and Costa is central to that, being what Conte calls his point of reference.

Eden Hazard and Pedro can float in those awkward inside-forward positions in part because of Costa’s role in occupying defenders in front of them. That is part of the reason why he has been fouled an average of 2.7 times per game, more than all but three other players in the league.

But what is most encouraging for Chelsea is that he has taken the kicks without undue complaint, his mind less on personal slights for which revenge must be taken than on leading the line and scoring goals. The canary looks in excellent fettle.