I think it would be hard to argue it was a "conditioning" thing.
Now, if you want to go back to 2015, after he's out for a month with a concussion, and he's essentially being told to take pills that have proven to lead to addictions issues, while being denied ability to use a much safer, arguably more effective drug, then yes, I could see him coming back from that and mentally being in a "less motivated" spot than he was prior to it. But I'm sure the writing was on the wall that he wasn't going to return after that season by that point already. And that's also extreme conjecture and really isn't supportable either.
I'm never going to be the guy who's out here pretending to be a Doctor who diagnosis NFL injuries and tells players whether they're mentally or physically capable of playing or not. You see clowns on the internet all the time who are like "well X player played with a torn labrum and Y player is weak because he didn't" and blah blah blah. Its all crap. Most of the time you don't leave these decisions up to players to begin with. And if I'm a player, I'm getting healed the best that I can, instead of delaying and risking not getting paid to clear an injury.
Not to mention that I didn't think Monroe was a stud when he was here to begin with. He wasn't bad, but I didn't think he was anything special. And so I'm not sure I want a player like him out there playing concussed or playing with a significantly shoulder or knee injury. Why do I think he would be better than the alternative we would put out there?