I can't see Manziel having the pocket skills to be better than most of the notable Qbs Mackus listed above.
He's too small, and NFL teams are going to make him stay in the pocket. He's just not going to be able to run around in the backfield like he did in college and create. I mean he'll be able to do it somewhat, and it'll be exciting, but he's just not going to do it as much as he did in college. He has to learn to read and get the ball out.