Facilities of your club can affect different actions so be careful to upgrade them when a construction is finished at a normal time.
-If your ground/grass level is similar with your club level there is a neutral effect. If it’s not upgraded, this gives a +1 to have an injury.
-If your medical center isn’t upgraded, that gives a +1 day in an injury of your player.
-If your training center is in a higher level than your club, it gives a ( -1) in a possibility one of your players get injured (besides that it gives a better training result).
There is an analogy between facilities levels and those actions.
This doesn’t affect opponent team players.