The Brazil-born Spain international was taken off in the 75th minute in yesterday's 3-2 win at Hull after setting up the opener and scoring the second in the first-half.
Mourinho fears the 26-year-old could have picked up another hamstring injury, but believes Loic Remy and Didier Drogba provide sufficient cover up front.
"He has this (hamstring) problem," Mourinho said on his leading goalscorer.
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Diego Costa was taken off against Hull with a hamstring complaint
We never cry about injured players
Jose Mourinho
"He tried to play the Champions League final for Atletico (Madrid, last season) and was injured again and again and again. He has this fragility.
"We know his hamstring is not a strong one. He works hard through the week to compensate the weakness he has there but the injury can come.
"If he is injured then we have Remy, we have Drogba. We never cry about injured players."
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