The 26-year-old suffered a reoccurrence of a hamstring injury against Stoke last Saturday and will miss this Sunday’s derby against QPR.
Mourinho initially ruled the striker out for two weeks immediately after the Stoke win and admitted he would not take risks with his recovery.
Loic Remy is likely to continue up front in Costa's absence but the Portuguese manager is confident the Spain international can still play a big role in Chelsea’s push for a first title since 2010.
“He will certainly be back before end of the season,” Mourinho said.
“We have eight matches to play, if he plays four it is normal.
“If he plays three it is a little bit below what I am expecting.
“If he plays five then it is more than what I’m expecting.
“I’m thinking about four matches.”
He added: "At this moment I don't want to think about Diego for two, three weeks. There's no point crying because a player is injured.”
