Diego Costa signed from Athletico Madrid in the summer for a £32m fee and has scored seven goals in four Premier League appearances but was only used as a sub last night for Chelsea’s Champions League clash with Schalke.
Mourinho said Costa was not fit to start after scoring a hatrick against Swansea last Saturday. A hamstring problem which resulted in Costa limping out of last season's Champions League final against Real Madrid recurred while he was on international duty this month with Spain.
The Chelsea boss told Sky Sports: "Costa has a problem," Mourinho said. "He came to us (from Atletico) and he was completely fine. He had a problem again in the national team, not with us.
"If he has one week to let the muscle recover he can start a game.
Playing on Saturday, three days is not enough for him to recover.
"It was not to protect for the game Sunday, it was because he was not in condition to start the game. Now we have the problem and now we have to resolve the problem the best we can, ‘’he added.
"He was not in condition to play. Sunday we believe he is. Next week against Bolton (in the Capital One Cup), for sure he isn't. We have to manage the situation until the moment he's completely fine."
The Chelsea boss will be hoping Diego is fit for Sunday's clash with Premier League champions Manchester City.
The Portuguese said Didier Drogba`s display was a "positive" one and replied when asked about his choice of continuing with Thibaut Courtois starting ahead of Petr Cech.
"I don't accept that you ask me about the goalkeeper every week," he said.
"For me it's just a decision and I can't say more than I say. What I say is that I have two fantastic goalkeepers, two of the best three goalkeepers in the world. And it's a fantastic situation for Chelsea."