I did a bit of a test awhile back, keeping a team at low condition to see if more injuries happen, it was more but not as much as I expected. It was on a team with the physical higher than other skills though... cannot suggest that enough, add to physical. The injuries aren't entirely random that's for sure, and I know you are frustrated that it's your best players, but are they also the players that are in every match? Do you use arrows on them? Are you going hard attack or defend? I think those things affect injury too.Besides too much training, of course.

See which guys are most tired after a match and try adding physical to them and taking off arrows if you are using them and see if it helps?