One reason may be your opponents tactics, which unfortunately you can do nothing about. It seems to be a common thing that a lot of lower level managers use "hard" tackling, which could account for the bookings for him and injuries for you.

Also, the ground structure can have a bearing. If you have low medical facilities, or low level pitch quality, these can also add to injuries. I'm not saying this is definately the case, but they could be a contributary factor.