The midfielder has been plagued by hamstring problems since his £12m move from Everton over the summer, starting only two games before getting hurt on England U21 duty in October.
But Mancini has earmarked him for a place on the bench at the Etihad Stadium and he hopes City will soon see some return on their investment.
There was also good news for Mancini on the availability of striker Sergio Aguero, who will return from a hamstring tear he suffered in the New Year's Day win over Stoke.
Mancini said: "I am happy because I am sure Jack will be an important player for us in the future, because he is young.
"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.
"Jack is back and Sergio is fit so we are now missing only Micah (Richards) and Maicon."
Mancini also admitted his pleasure at having skipper Vincent Kompany available after his potential three-match ban for being sent-off at Arsenal was rescinded.
He added: "I am very pleased because I think it is correct, like this. The referee managed the game very well but, in that situation, probably he didn't see great.
"From his position he thought it was a red card but I think that tackle was not a red card."