Yeah, it doesn't seem to be that simple. Apparently there are many other factors involved, such as the players above you, your ratings, your goal achievements and timing. Probably other things that I haven't thought of yet. And as of right now, I haven't figured a way of triggering a "call up".
The logic seems erratic as many have differing results w/out a clear motivating factor behind it.
Edit:
Yeah, I just simmed through a full month of my backup saved file and found that even if I injure all of the players that would effect my players positions. (3rd base w/ 2nd position as IF) it does nothing.
I injured Cabrera, Fielder, SS, 2nd basemen and the only two infield backups. I couldn't injure anyone else, it wouldn't let me. The team then proceeded to put outfielders into those positions and played w/ no backups for the entire month. I gave each player a injury that would keep them out the entire season.
During that stretch I successfully succeeded at all of my goals, batted at a extreme rate of .479 and hit 7 HR's. Never did I even get a sniff of a call up.
My only thought on this is that I am not on the 40 man roster and this is why I'm stuck in the minors. However, they didn't call up any of the other players either that was on the 40 man roster, so I don't know what to think about that.