Yeah, if your phys/mental is lower than your opponents, sadly your % of injuries skyrockets. It's why it's such an valued stat bank. Basically it's fitness and strength vs tackling and aggression minus whatever other modifiers are in play (recently healed, opponent hard tackling, field condition, etc.) It's why my pitch was the first upgrade I had to lvl 3. I hate having to baby star players with playing time but risk vs reward and all that.