Diego Costa
Costa's fitness problems likely to rule him out of Old Trafford visit. Getty

Diego Costa's chances of featuring against Manchester United on Sunday have suffered another blow following reports the Chelsea striker is struggling with illness.

A number of reports, including from the Daily Telegraph and the Daily Mail report that the 26-year-old, who has not trained with the Blues since returning from international duty, has been battling a mystery stomach bug that forced him to go to hospital this week.

Costa, who has struck nine goals in seven league appearances for Jose Mourinho's side this season, was already a doubt for the trip to Old Trafford after missing games against Crystal Palace and NK Maribor with a groin problem.

The £32m signing picked up the knock while on international duty with Spain for Euro 2016 qualifiers against Slovakia and Luxembourg, a situation that has prompted criticism from his club manager.

"It's up to them, to take of players, or be selfish," Mourinho recently told the press, referring to national football associations. "I don't tell players not to (go with their national teams.) I like to stimulate players to go – I like players to go.

"Sometimes you have national coaches and medical departments that want to establish a relationship and they are interested in the players, their physical condition, their habits.

"Others they simply don't care. This is not something we can resolve. I cannot ring the federations and say I want to speak to the coach, or the doctor wants to speak to their doctor. It's something I cannot resolve."

Mourinho may be forced to hand a start to 36-year-old Didier Drogba after Loic Remy limped out after 16 minutes in the 6-0 win over Maribor on Tuesday.

The Chelsea boss spoke pessimistically of the Frenchman's chances of starting against United on Sunday but the striker was quick to play down the severity of the knock and hopes he will be past fit after undergoing a scan on Thursday.

"We will see with the scan," he said on Wednesday. "I really hope it is not too bad. Fortunately, I stopped playing because I felt something. I don't think it is too bad, but I felt something. My groin."