City swooped for the former Wolves midfielder in August.
After Nunes' performance in victory over Nottingham Forest, Guardiola said: “Really good.
“Sometimes he has to stay more calm in the position, but he has the dynamic.
“In the game today, give the ball, dribble, and drop the opponents down.
“He looks like he's never tired, because his actions over 15, 20 metres – but we knew it from Sporting Lisbon where he has done it, as well as at Wolves.
“We asked [Julen] Lopetegui, who is a friend of us, how is he, and all the opinions were really good.
“He has runners, good finishing, defensively [he] can play in two or three positions.
“The last two games he played against Red Star and today, he was a key point - really, really important.
“He's an exceptional player, exceptional."