Slimani has been plagued by a reoccurring groin problem this season after his £30million move from Sporting Lisbon, and has yet to start a game for City since returning from CAF2017 with Algeria.
He came off the bench at half-time at Swansea, last Sunday, and his presence has helped City improve.
However, Ranieri said he has felt groin injury again in training and will he will not be ready for FA Cup trip against Millwall, on Saturday, and possibly Champions League last 16 first leg tie against Seville next Wednesday.
"He has felt something in his groin," said Ranieri, adding "For these matches he is not available”.
"It is something strange but we must see. We must do the best for him because he is an important striker for me”, Ranieri added.
"He doesn’t need surgery, we must make the right treatment and he will improve", Ranieri revealed.