= new reply since forum marked as read
Don't take that the wrong way... 
The case of Hoyer winning the job over him was due to him struggling to learn the playbook. Difficult to go from a spread offense w/ no playbook to a complicated Shannahan one. -- http://www.si.com/nfl/2014/08/20/johnny-manziel-brian-hoyer-cleveland-browns
He actually moved the offense more efficiently than Hoyer, granted it was against 2nd string but you don't want to throw someone w/ a steep learning curve to the wolves against the Steelers, Saints, and Ravens.
On edit - This author raises a good point, at Texas A&M he had 2 All-American pass protectors he hasn't been tested w/ the pass rush.
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He has two good tackles in Cleveland—Joe Thomas and Mitchell Schwartz—but he's seeing defenses that are fast enough and strong enough to finally pressure him. Washington defensive coordinator Jim Haslett wasted no time in getting his edge defenders after Manziel.
Look at how Washington played him pre-snap. There are eight players near the line of scrimmage—something he saw very little of in college. Only Alabama head coach Nick Saban routinely tested Manziel in this way at A&M, and never with the size and speed that the NFL can unleash on a quarterback.
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his is where Manziel's biggest struggles have come from thus far. Mentally, he's unprepared to look at this sophisticated defense and have a game plan pre-snap for what comes next.
He must know pre-snap who the "Mike" is, what the coverage is, if the cornerbacks are playing press or off, where the free safety is lined up and where his hot read is. That's a lot for any rookie quarterback to digest, and Manziel is struggling to make those reads.
That doesn't mean he can't pick it up and figure it out but right now he's mentally drowning before the snap.
http://bleacherreport.com/articles/2172296-breaking-down-johnny-manziels-game-potential-after-preseason-week-3