There was an interesting and important topic on injuries with jdb in the forums a while ago with him explaining injuries. jouameng had a link to this in his Parody league.(how he dug this up i'll never know lol) Great research.
Here is the link to the topic on the general foums. It is a must read for newer GM's.
https://paydirt.myfootballnow.com/community/1/612?page=3#12684