The Argentina forward, a club-record £38million signing, enjoyed a stunning debut in City's 4-0 win over Swansea, scoring nine minutes after his 59th-minute arrival, setting up another for David Silva and then hitting a 30-yard shot into the net in injury time.
"I think he needs to recover as he only finished the Copa America three weeks ago," he told Sky Sports 1.
"He needs another two to three weeks to be 100%, maybe. For me he's a fantastic striker, young, and he's going to be a fantastic player for us."
Aside from Aguero, Mancini was happy to kick the season off with a win - the only home victory anywhere in the Premier League so far.
"I am delighted with this performance," he added.
"In the first half Swansea played very well. We had some problems, then we pressed them and had some chances and in the end I am very happy."