The former Everton player has failed to launch himself into the City first team due to a number of injuries since moving to the club in the summer.
"I am happy because I am sure Jack will be an important player for us in the future, because he is young," BBC Sport quotes Mancini as saying.
"If we have solved his problem, his hamstring injuries have been often, he will be an important player. Tomorrow he will be ready, but probably for 20 minutes, no more."
City currently sit seven points adrift of leaders Manchester United.