A QB can get hit just as hard or harder staying in the pocket when he's oblivious to his surroundings. Tannehill has probably taken more shots in 9 games this year than Manziel has taken his whole career. Tannehill has only missed (most of) one game in his career, and it happened when he was in the pocket. How does Russell Wilson do it?
I'd be shocked if Manziel isn't great unless his arm just really blows. Poise, vision, decisive action are attributes that translate to any arena.
Tell me what about Manziel indicates that he would fail. You think he wouldn't put in the work? I think he would because his ego would demand that he play well, and that requires putting in the work. Tannehill gets praise all the time for his smarts and work ethic, yet he makes mental mistakes all the damn time.