This is a quote from a ticket I had some time ago. In my case I had promised the player minutes and when he got hurtI didnt play him until 80% and he started complaining about minutes.
"Just so you know, there is no higher risk of injury by playing someone at less than 100%. There is just the reduced effectiveness."
On a side note to the promise thing, if you promised minutes and were afraid of the reduced effectiveness, he will be expecting to play once he is back to 60% again. In my case, I sat player until 80% and he started complaining about minutes. His stats showed over 15 mpg, b/c it only averaged the games he played, but once he passes the 60% internally he is counting those games and it does not show in his stats cosmetically, so he will complain even if the stats show you are holding your end of the promise.