You might be unlucky. I usually try to buy the generated players to avoid the insta-injury...
I think a lot of people who are selling players (especially good ones!) is because they are injured for too long to keep, and since there's no virtual doc giving a physical the only way you find out is when they instantly get hurt. (also you will often get them with 0 moral and fitness)

Letting them sit a day might help. I try to not play guys I buy from other managers the first day or at least many hours from when I got them to try to minimize insta-injury and it mostly works for me.