He may well have chosen to withdraw his services to the national team. I get that.
The remainder of your possible explanation, may even have some merit. However I cannot recall any national team, defining the duration of any injury, much less a muscular one.
Supposition apart, I am waiting for the Blades management to clarify this unusual injury claim, and resolve any issues raised by it.
If a player is injured, he is injured. It’s hard enough for the parent club to define the length of absence, so why does it seem acceptable for his national bosses to do something the players club would struggle to provide?
Serbia are not a world renowned authority on injuries, or their duration! This is why I suspect that something odd or unusual has happened.
See what the club say first, before the supporters re-evaluate the clubs entire response!