Former City captain Tevez spent six months on the sidelines last season after a row with Mancini, but returned to help the team win the Premier League title.
Mancini thinks that the Argentine striker will be instrumental as City attempt to defend their championship this season.
He said: "I am happy with Carlos, he's worked really well since we started training. His attitude is excellent, now he must continue like this.
"It was hard for him last season because he had six months out, so when he came back he was a long way behind.
"What happened last year with Carlos is forgotten, in the past, now he has got a full pre-season and can be important for us."
City play their first competitive match of the season on Sunday when they take on Chelsea at Villa Park in the Community Shield.