I was watching the game - this is not what happened in real time - it is revisionist history provided to us by the media. I don't like Michigan at all, so I can't believe that I am defending Hoke here.
On the play in which Morris was concussed (looks like he probably was), he laid on the field for about 3 seconds after taking a vicious hit (which drew a personal foul penalty). He then popped up and looked fine in real time. You slow it down and he looks woozy, but in real time, while watching the game, it was far from clear that he had been concussed.
On the next play Morris threw a bullet that was not caught by his receiver. After the play he looked like he was out of sorts even though he had not been touched. Hoke pulled him immediately and he went over to the trainer who took his helmet and started talking to him. Gardener came in at QB.
Gardener ran three plays and then his helmet came off on a run. Morris went back into the game for one play, a handoff, and then pulled. The trainer gave him his helmet back. The camera crews were following the 3rd string QB as he attempted to find his helmet, but that was inconsequential as Morris went onto the field immediately. He ran the play, then Gardener went back in and Morris never saw the field again. Someone took away his helmet for the rest of the game.
Now, I have no idea why the trainer gave Morris his helmet for that play. In today's college football, the trainers and medical staff make these decisions, not the coaches. So it might have been on Hoke that he let him play a single play immediately after the hit, but the other play that he ran is on the medical staff, IMO.
Most of this story is written with the advantage of slow motion cameras and multiple camera angles. Also, the head coach is coaching the game, not watching any specific player on every play. To assume that Hoke knew that he was injured and left him in is typical of the media. Makes for a story that people will read and react to. But it does not reflect the reality of the situation as it played out on the field.