Pep Guardiola praised Jeremy Doku after the Belgian forward scored his first Manchester City goal to inspire a gritty 3-1 win at West Ham on Saturday.
Guardiola's side trailed to James Ward-Prowse's first-half strike at the London Stadium.
But City fought back to level within seconds of the restart when Doku cut inside Vladimir Coufal and drilled in his first goal since his close-season move from Rennes.
Bernardo Silva and Erling Haaland scored in the final 15 minutes to ensure City maintained their perfect start and stayed top of the Premier League with a fifth successive victory.
Guardiola said he had not expected such an immediate impact from the 21-year-old Doku, who made his debut against Fulham before the international break.
Guardiola watched that game remotely from Barcelona whilst he recovered from back surgery, but since returning to Manchester has been impressed by the progress Doku has made.
"I was in Barcelona, I had the feeling that he played a little bit shy (against Fulham)," Guardiola said.
"He'd just arrived, had two or three days training, with a team that won the treble. Maybe he was a little bit in this way.
"Today was from the first minute incredible, aggressive, great determination, arrived many times to the byline. Many good things he's done. Really pleased. We thought the quality was there and today he started to prove it.
"When you buy a young player like him for many years (contract), we have to be patient. I didn't expect the second game for City to play like he did today. The way he played today, I don't remember from a long time ago something like that."
City are chasing an unprecedented fourth consecutive Premier League title after winning the treble last season.
And, after fighting back against West Ham, they will kick off their Champions League title defence against Red Star Belgrade on Tuesday in good spirits.
"I would say West Ham had more chances in the second half than the first," Guardiola said.
"We created against a team that defends so deep and really well. We created a lot of chances in the 90 minutes.
"I'm so proud of the team. It's great win for us, for the problems (injuries) we have in the squad. It doesn't matter, the guys always respond well."