Exactly what we didn't need atm... how do we even manage to always get this unlucky is beyond me, and the timing of it above all couldn't be worse, we have lots of experiments to do with the squad and having your main LB/LWB and even makeshift LCB available would have made a world of difference, but it is what it is right now obviously and we got to deal with it, going to probably be counting on Pellegrini to do his best there for the time being and to eventually step up his game if possible, IMO overall this could in fact be a golden opportunity for him to earn his place so we'll at least see soon enough if he's ready or not for it.
Hopefully though it's just something minor in AS's case and will not take longer than predicted to recover.
I feel like he has one of these per season at least? Muscular players can get them any random time, but it's not exactly uncommon for small strains to happen during pre-season when players go from nothing to intensity again. The short break will also have a factor, depends on the player. Remember Alex Sandro was ran into the ground last season.