"I think it is a hamstring injury. Maybe it is not so bad but I don't think he will be ready for Chelsea," said the Italian.
Mancini admits City will need to produce another battling display against the Blues, who have lost only one of their last five games.
"Chelsea are in a good moment, they play well and it will be difficult," he said.
"It will be a good game, probably, but it will be different. Semi-finals are different from championship games."