Costa lasted just 11 minutes as a second-half substitute in last weekend's 2-1 victory over Stoke City before pulling up with a hamstring injury yet again.
The Spain international is expected to be missing for around two to three weeks, but Mourinho stressed that he still has a big part to play in Chelsea's bid to win their first title for five years.
"Certainly he'll [Costa] be back [before the end of the season]," Mourinho said ahead of Sunday's London derby at QPR.
"I think we have eight matches to play, if he plays four it's normal. If he plays three he's a little bit below what I'm expecting and if he plays five he plays more than I'm expecting. So I'm thinking about four matches.
"I'm not concerned about his injury, I'm concerned about the match on Sunday. I don't want to think about it for two or three weeks until he is fit. You never see me crying because a player is injured. I don't want to think about it too much."
Costa is Mourinho's only concern, with John Obi Mikel fit and Cesc Fabregas set to play in a protective mask due to a broken nose.
Fabregas and Nemanja Matic are one booking away from a two-match suspension, but Mourinho will not hesitate to play the midfield duo in the hotbed of a derby at Loftus Road.
"They will both start. It's always the same, we don't want bookings, we try not to get yellow cards during the match," he added.
"If they have to make a foul for a yellow card, these fouls you have to do it. The way we play I think it's not easy for our players to be booked.
"The game is a three-points game, we need points, we need victories. It doesn't make any sense to think about some matches and not others. This match is a three-points match and we have to play to our maximum to get them."
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