London - Jose Mourinho revealed Diego Costa is currently unable to play three games in a week after the Chelsea striker suffered a hamstring injury while on international duty with Spain.
Costa has made a blistering start to his Chelsea career after a £32 million move from Atletico Madrid and his impressive haul of seven goals in four games, including a hat-trick against Swansea City on Saturday, had lifted his new team to the top of the Premier League table.
But the Brazil-born star sustained a hamstring problem during Spain's friendly against France earlier this month and was forced to miss their Euro 2016 qualifier against Macedonia as a result.
Now Mourinho says Costa's injury means he has to be treated with extreme caution to avoid aggravating it and potentially losing him for a long period.
The 25-year-old was left on the bench for all but the last 18 minutes of Wednesday's 1-1 draw against Schalke in the Champions League, with his appearance coming too late to break down the Germans' obdurate rearguard action.
"Costa has a problem and he can't play at this moment three matches in a week," Mourinho said.
"After playing on Saturday, three days is not enough to recover. It was not to protect him for the game at Manchester City on Sunday. It's because today he was not in condition to start the game.
"He came to us in the summer and he was completely fine. He had a problem with the national team, now we have the problem and we have to resolve it as best we can.
"I don't know how long this will be the situation. We have to manage it.
"He went to the national team in perfect condition and came back injured.
"It happened to Diego and happened to players in so many other national teams. I don't need to be critical, it is just the reality."
With Costa unable to bring his potent combination of predatory finishing and powerful approach play to Chelsea's forward line, Mourinho's team were destined for a frustrating opening to their European campaign.
They had the perfect start when Cesc Fabregas scored his first Chelsea goal, the Spain midfielder slotting home from Eden Hazard's clever reverse pass in the 11th minute.
But Fabregas wasted a golden opportunity to double Chelsea's lead before half-time and then Didier Drogba, drafted in as Costa's replacement, failed to make the most of the first start of his second spell at the club.
The Ivory Coast forward clipped a good chance wide and Schalke captain Klaas-Jan Huntelaar made him pay with a clinical 62nd-minute equaliser from Julian Draxler's pass.
Eden Hazard and substitute Loic Remy both came close to winning it for Chelsea in a frantic finale but Schalke held on for a deserved point.
"Fabregas had a good chance in the first half and Didier in the second. We kept trying to score and then we conceded on a counter-attack situation," Mourinho said.
"We made two mistakes. One is not to make a foul and the other one was the fact that I told my players 20 times that Draxler wants to come inside on his right foot.
"But after that only one team tried to win. We had so many chances.
"It's a good point for them and a point for us, but not a fantastic point."
After watching his side fail to win any of their opening three Bundesliga fixtures, Schalke coach Jens Keller was ecstatic with his side's display.
"When I went into the dressing room I told the team how proud I am. If they can play like that against a world-class team there must be something right," he said.
"Yes we need a bit more consistency but I just hope we can get a few of our injured players back because that will help us a lot."