GETTY Diego Costa could leave Chelsea and return to Spain in the summer

The Spain striker has had a torrid season as Chelsea have struggled this year, but last night he said he was sorry that he threw a bib at manager Jose Mourinho in a fit of pique and insisted he has an excellent relationship with his manager. Costa was restored to the first team by Mourinho after two unhappy games on the bench and Chelsea steadied a rocking ship by reaching the last 16 of the Champions League on Wednesday thanks to a 2-0 win over Porto. Though he did not score and in fact wasted several good chances, Costa worked hard in his best performance for some time. But afterwards he failed to knock down the suggestion he could return to Atletico at some point. Chelsea do not want to sell the £32million striker they only signed from the Spanish club in the summer of 2014, and whose 20 goals in a sensational first season helped them win the title last year.

But asked about the possibility of following full-back Filipe Luis, his close friend who returned to the Spanish capital after one season at Chelsea, the 27-year-old said: "You never know - look where Filipe Luis is now. Life is full of surprises. "I always wish Atletico the best and I always follow them, support them from afar." Chelsea are looking for striking reinforcements in January rather than selling, with Atletico's Antoine Griezmann, Shakhtar Donetsk's Alex Teixeira, West Brom's Saido Berahino and Jonathan Calleri of Boca Juniors all on their hit list. With Griezmann unlikely to be available before the summer, it could be that a swap deal with Costa occurs then. Costa, who admitted he returned from the summer overweight, has had an unhappy season, scoring only four goals. He has admitted to being upset with the FA for handing him two retrospective three-game bans during 2015.

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Mourinho praised Costa's efforts on Wednesday, but it looked as though their previously tight relationship was crumbling after the striker was dropped to the bench at Tottenham two weeks ago, and when not brought on, hurled his bib at his manager on the bench. But Costa said: "The bib incident is in the past. I made a mistake, but my relationship with Mourinho is amazing, we get on really well. There are things you need to control, but the bib thing was an impulse. "I have to improve. The start of the season has not been good for me, not in how I have played, nor in goals. "The truth is Chelsea haven't been in a great moment, above all losing the other day [at home to Bournemouth]. It was important to win against Porto to make the fans happy and to recover our confidence. "I know I have to be better to go to Euro 2016 with Spain. I have to work hard and fight, but I know I have time to improve and I hope I will."

The win, in front of owner Roman Abramovich, took some of the pressure off Mourinho, at least until Monday's tricky league game at leaders Leicester. Defeat there against former Chelsea manager Claudio Ranieri would ramp up the speculation over the manager's future once again. The win over Porto was Chelsea's best performance of the season, but midfielder Oscar warned they will have to start transferring their impressive European form into league points. "When the team play well we win games. It's difficult to explain why our performances can be so different, but we need to play like that in the Premier League," he said. "We need to come back to form in the League. We need to focus and play like we did against Porto." Willian, who scored the crucial second goal against the Portuguese club that meant Chelsea finished top of Group G and so should have an easier prospect in Monday's draw for the last 16, said: "This result can be the one that turns things round for us. But we have to go to Leicester to win. No doubt - to win."