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.
"He's decent," said Hart. "He's come in, £38million is a big price but it's £38million well spent. We've not seen much of him so far, the manager has kept him well protected."
Captain Vincent Kompany added: "He's come in with a surprise. The great thing for me was that he was defending too. That's important."
"There's a great bond here. Sergio, Gael and Stefan are feeling the love," Hart said.