Chelsea striker Diego Costa ruled out until May with hamstring injury

Chelsea will be without top scorer Diego Costa for the crunch Premier League clashes against Manchester United and Arsenal after the Spain striker was ruled out until May with a hamstring injury.

Costa was replaced 11 minutes after coming on as a half-time substitute in the 2-1 win over Stoke City and will at least miss the club's next four matches, not returning before the visit of Crystal Palace on 3 May.

The 26-year-old is out of the weekend's London derby against Queens Park Rangers, but his absence will be felt hardest in the back-to-back games against third-place United and second-place Arsenal, both of whom have faint hopes of staging a late title bid.

"Certainly [Costa] will be back, sure," said Mourinho. "I don't know when, but we have eight matches to play. If he plays four it's normal. Three is a little bit below what I'm expecting. If he plays five it is more than I am expecting, so I am thinking about four matches to play."

John Obi Mikel and Cesc Fabregas are both fit to start at Loftus Road, but the latter along with Nemanja Matic are a booking away from being suspended for the visit of United on 18 April.

However, Mourinho says neither player will play differently against QPR, despite the fear of joining Costa on the sidelines for the game against United.

"I won't leave [Fabregas and Matic] out, they will both start, it is always the same," he said.

"We don't want booking, we try not to get yellow cards and try to play football. If they have to make a foul for a yellow card, these fouls they have to do it and another one plays, but the way we play it is not easy for our players to be booked."

FC Tokyo president Naoki Ogane confirmed in mid-week that Chelsea have bid £4m for Japan midfielder Yoshinori Muto, news that comes just six weeks after the club confirmed a £200m shirt sponsorship deal with Yokohama Rubber.

But Mourinho has denied there is a link between the two deals and says Chelsea would not be interested in Muto, who has netted three goals in the J-League this season, unless he was good enough for the Premier League.

"We would never bring a player if the player has no capacity to be in a Chelsea squad," Mourinho stated. "I don't think about players, I don't think about next season. Chelsea will never sign a player who isn't good enough to play for the club. We will also always look at Financial Fair Play."