I think he needs different training or more training to some of the others. He looks unfit. Think; not all players with the same training will reach the same level of fitness and body fat percentage as each other. There will be variances in the squad and some players will need different or more training to reach the required level of fitness and fat to play in the Prem. Some players might not get to that level of fitness despite being better than 90% of the rest of the league.
I noticed last season that de Bruyne, Yaya, Kun and Nasri seemed to be carrying too much fat. de Bruyne and Nasri would look knackered in the second half of games (noticed this of Nasri for a few years), Yaya would after five minutes, Kun would just drift out of games but not look tired. Even look at Fernandinho and Fernando in the second half of two of the biggest games of the season - Madrid and Arsenal at home - blowing out of their arses as the opposition came on strong later on.
The fitness of this side was very poor last season. Nasri's seems to have been for a good while. I'm certain Guardiola would not stand for this (maybe that's why Mancini had the falling out with Nasri back in 2013-14?) and will get these lads fitter and carrying less fat (not the Ferns obviously but they still aren't fit!)
Plus I think we are far too harsh on him performance-wise. There's no way he has seven bad games in ten. I don't know what people see sometimes. He's inconsistent, yes, but not that bad and what he brings to the team is very high quality. And in almost all games (seven or eight out of ten) he'll bring excellent passing. When he didn't play last season look at the sheer amount of times we'd give the ball away in ridiculous situations: de Bruyne, Sterling, Delph being prime examples. Nasri keeps the ball. Some people says he slows the play down, I think that's because he's ahead of his teammates in where people should be stood to receive a pass, he's a passer he wants to pass and too often players in our team stand still and don't allow themselves to be passed to (a huge problem under both Mancini and Pellegrini, that). His fitness might be a factor in that slowing down too much too.