Neither of us know, but I'm not in the business of presupposing either as many on this thread are. How many times have we seen players given time off because they've suffered a bereavement? By your metric that player should still be OK to perform because he's still physically fit.
How many times have Mancini and Guardiola mentioned how important mentality is in football? If a persons state of mind is not right, rarely will you see them perform to their best....... that is unless you can prove different?
Sanchez's mentality is probably not right because of the huge disappointment he suffered in the summer. Are you seriously trying to claim he has no right to allow this to effect him? Could you help feeling a surge of grief if you lost someone close (heavens forbid)? Just imagine if this did happen and those around you started accusing you of downing tools, losing you ability overnight, being a malingerer, or deliberately not trying, how would you feel?
Sanchez has enough deposited in the bank for me to be excused if he's not feeling his usual perky self. Perhaps he just needs a change of scenery eh?