Chelsea fear Diego Costa will be ruled out of their trip to Manchester United next weekend after the Spain striker was finally forced to rest the troublesome hamstring that has dogged him all season.

Costa, right, returned from international duty last Monday and spent the week undergoing treatment on the thigh complaint and José Mourinho was unable to pinpoint when the 26-year-old will be fit enough to return.

“I have no idea,” he said. “The only thing I can say is that, after the Arsenal game, he went to the national team. He played two big matches against Slovakia and Luxembourg, and he came back in conditions where he’s not available to play for his club.”

The forward played all but eight minutes of those Euro 2016 qualifying games, despite Chelsea having made clear their concern over the state of his fitness, and sat out his team’s victory at Crystal Palace on Saturday. Mourinho takes his team to Old Trafford on Sunday with Costa’s involvement already in considerable doubt, even with a week of rest ahead, including missing Tuesday’s Champions League game against Maribor.

Asked if he had sought to contact the Spanish Football Federation since Costa’s return, Mourinho added: “For what? For what? To spend money on phone calls? No. I think Diego will be in great conditions again in mid-November, to be back for the national team. That is for sure. Our medical department will take good care of him and he will be in perfect condition. He will play for me when he has free time from the national team.”