So, three years ago, Trent suffered a concussion that was so severe that, to this day, it's still causing him to be a perform like a well-below-average (marginal) running back. But, at the time it happened, the medical staff didn't know see any effects, or at least no effects significant enough to put him on IR? And, he was able to "play through it"?
I'm with RTR91. Unless it's completely a mental barrier he's having trouble getting past, I'm having trouble buying the supposed concussion as the root of Trent's struggles.