The French midfielder was brought down by Mapou Yanga-Mbiwa in the latter stages of the second half, with City leading by a single goal at the time.
And, after offering a bleak outlook regarding the player's knee injury, City manager Manuel Pellegrini questioned why Yanga-Mbiwa was not shown a red card for the challenge.
"Samir Nasri has a problem in his medial ligaments," he said. "We'll see tomorrow how serious it is, but I think it is a serious injury.
"I think it was a very unfair kick by the player of Newcastle.
"It was a direct red card. I do not understand why he was not sent off."
Edin Dzeko gave City the lead early on, before Alvaro Negredo made sure of the three points in stoppage time, but the major talking point came when Cheick Tiote had a superb strike controversially ruled out for offside, denying Newcastle a first-half equaliser.
"I think there was a player offside when the other Newcastle player kicks the ball. It was not a valid goal," Pellegrini remarked.
"It was a very big result, because it's a very difficult stadium to play in.
"We scored two goals and we had two or three more chances to do it."
The result, which saw City keep an away clean sheet for the first time since a League Cup victory at St James' Park in October, lifts Pellegrini's men top of the Premier League, a point clear of Chelsea.
