A player you call up will lose his face and just become a generic bald guy. So say the dude had a beard, hair, and noticable facial features, he will have none of those when called up. He will just be a generic bald guy. I'm not sure if you have the game, but if you do just take a look at one of the umps and that's how he will look(although the player will have the right skin color).
I only experienced that glitch in spring training though, I've called up some guys during the season so far and I have not had the issue. I'm just going to avoid spring training because almost half my 40 man roster was generic. What I experienced in spring training was that most of the guys I called up were bald and generic, but as I played more games people on my team who had been on the mlb roster the whole time were turning into bald generic people.