Manziel lacks the patience, and the discipline required to be an NFL quarterback. Quarterbacks are required to make precise, and careful decisions from within the pocket, yet if something isn't there for Manziel, he turns on his jets and scrambles for 10 to 20 yards, and runs over a defender who's in his way of crossing into the endzone.
Sure, as long as he is in college, this is fine; he can run all over the field in college, but not in the NFL. In the NFL, if a passing option isn't there, he isn't going to be able to run out of pocket, and scramble for 30 yards like he's used to, the defenders are bigger, stronger, faster, and smarter. He is going to have to establish patience while in the pocket, besides, he's only 6'1", 209 pounds, he is bound to get hurt if he takes punishment.
Opposing defenses in college:
The assumption can be made that one wouldn't need to mention the difference between an NFL defense and an NCAA defense, one is professional, and one isn't. But that isn't the only factor; due to Manziel's scrambling ability, opposing defenses in college have to prepare for his break-out speed, this partially takes away the focus they can assert on Manziel's passing options. This creates easier passing lanes, as defenders are awaiting him to scramble out of the pocket.
Need one to mention the lack of success the read-option plays in the NFL nowadays? Sure, in 2012 it worked like a charm, defensive coordinators were caught off guard, and the likes of Robert Griffin III, Colin Kaepernick, and Russell Wilson helped utilize it; now, it isn't used as much because it can be stopped, you simply cannot implement a college style of play in the NFL.
Manziel is 6'1", there might be a concern about his ability to see downfield over offensive linemen, and the defensive linemen trying to take his head off, a defender of J.J. Watt's skillset could possibly have a field day with Manziel, finding it fairly easy to bat down passes.
Of course we have quarterbacks like Russell Wilson and Drew Brees who have no problem putting passes where ever they want them to go, but it can still be considered a cause for concern, although not a big one.
Struggles with LSU's defense:
In Manziel's two matchups against LSU, the longest rush attempt they've allowed for Manziel was a 14-yard run, out of a total of 29 attempts. Manzie has accumulated the following stats against LSU(two games): 45 of 97, 500 YDS, 45.4%, 1 TD, 5 INT, 29 rushing attempts, 81 yards, longest run: 14 yards.
LSU was able to limit Manziel from his pocket escapability, and forced him to stay in the pocket -- he did not do well when he was forced to become a pocket passer, and in addition, this was against a college defense. This isn't just one bad game, this is two bad games against a defense that knows what to take away from Manziel, need one to mention the six players drafted from LSU's defensive unit? The same six players that played against Manziel in 2012. Imagine what an NFL coordinator and his defensive troops could take away from Manziel's game. This is a cause for concern of his pocket presence.
Manziel also doesn't have experience running a pro-style offense, some will argue that Robert Griffin III didn't either, yet he was able to amass 20 touchdowns, and only five interceptions, but the Washington Redskins built their offense around the rookie quarterback. They were able to utilize the zone read option; this is no longer efficient in the NFL. This is another cause for concern as he makes his decision on whether or not he will be entering the 2014 NFL Draft.