I think there are several things that go into injury likelihood, actually:

I think physical stats (fitness, strength) I like mine to be as high or higher than primary skill stats.
Training/Matches in the previous server day.
Yes, pitch. I mean it. Look at the last page of my team showcase and tell me it doesn't make an argument for it. Go ahead.
Condition, morale, and whether or not there is an arrow or OOP.

I *rarely* get unexpected injuries. I just PTed the heck out of them, and EXPECTEDLY the guys with less Fit/stam to skills and older guys got injured.