Kelvin De Bruyne's injury is one that'll last a while and may likely cause him to loose from upon recovery, he'd need some time off to get back himself and his agility to match Guardiola's football, he's a quick picking player and he's going to be able to bounce back, no doubt City misses him as he's got this special effect he gives to the team, a few things has changed since his absence and his absence is felt in a minut level.
Al Nassr got their eyes on him and no doubt it's his decision to go there or not, even though he'd cause a lot of impact in the team too, Al Nassr move won't be a bad option though but the guy has still got some displays to do, his contract with City will expire soon and then we'd know his way forward in the footballing journey.
You are right Kelvin de Bruyne injury has taken longer than expected because when he had the injury I was thinking that before this time he will be back in the field but on the contrary is still going to take more time before he could completely recovered. however it was very quite unfortunate that Manchester City has to play several matches without there key player.
So perhaps just as you said even if Kelvin de Bruyne will come back, he may not really be in a good shape to perform the way he normally do because he has stayed so long that the possibility of playing the way he normally do will not longer be guaranteed on less he starts training to regain his fitness.