I think Durability effects the severity of the injury.
If a 90 DUR sprains his ankle, his Health might drop to 85. He could improve that Health rating by 5-7 points per practice. You probably wouldn't notice a change in his game performance.
If a 10 DUR has the same injury, his Health might drop to sub-50. From what I've seen, they also will improve their Health rating by the same 5-7 points per practice. But when the Health rating drops that low he doesn't practice (leading to a possible overall ratings drop) and cannot play in a game until 60+