The defender made his first start in six months for the Blues following a serious knee injury.
However, Mancini believes that the full-back should have been in better physical shape for the match at the Etihad Stadium.
"I think that he needs to work a lot," the Italian told reporters. "I am happy because he comes back to play but I thought that he was in better form.
"After six months, though, it is impossible to play well for 70 minutes. He was tired but it is important that he came back to play."
City are back in action on Sunday when they make the trip to Tottenham Hotspur.