They should have done what the Jaguars did with Bortles: announce to the world that Manziel was going to sit on the bench for a while.   Then, whatever he did, it would look like success.   Now, after they pushed him into a battle for the starting job before he even knows the playbook, anything he does looks like failure. 

Some people seem to think you can't ruin a rookie QB, but you can.  You can absolutely ruin him.   Push him in there before he's ready, and make him an object of ridicule.  Manziel is obviously immature, and that makes what they have done with him even worse.  I doubt he will handle this very well. 

He should have sat on the bench and learned the playbook, and how to be an NFL QB.   Then, when he is ready enough to have some success, and can't fail to win the job, then put him out there. 

---