From the Dev. Chat on player ratings:
Durability (du): refers to how a player deals with injuries - the higher a player's durability, the less likely they are to sustain a severe injury and when they do get injured, how quickly they'll be able to return.
and later in the same chat:
Injuries are random but there is an inverse relationship between durability and injury severity - the higher the durability, the lower the chances of a *severe* injury and vice versa.