But the veteran Pittsburgh Steelers quarterback has the same questions as NFL coaches and general managers who have been evaluating one of the most discussed and polarizing players in the history of the draft.
"We've seen some of the scrambling, jumping, throwing, flipping type things he can do and it makes your jaw drop because it's special stuff. Does that translate to the NFL? I don't know," Roethlisberger said Tuesday on 93.7 The Fan. "The guys in the NFL are all bigger, stronger and faster."
Roethlisberger has absorbed his share of punishment in part because he has a penchant for extending plays. But Roethlisberger is listed at 6-foot-5, 241 pounds while Manziel measured in at just under 6 feet at the NFL scouting combine and tipped the scales at 207 pounds.
Roethlisberger said new Steelers defensive assistant coach Joey Porter, who is fifth on the franchise's all-time sacks list with 60, asked him recently if he thought Manizel would hold up physically in the NFL.
"I looked at Joey and said, 'Joey do you think if you got a good hit on him that he would be OK?' He goes, 'No, not at all.' That's my question," Roethlisberger said. "You better be really athletic or get down or be big. I think he definitely has a lot of upside but we'll just have to wait and see."
Manziel, the 2012 Heisman Trophy winner, had a storied career at Texas A&M and torched Alabama's vaunted defense in consecutive seasons. But opinion on him is all over the place, with some mock drafts having Manziel going in the top five of the draft while others have him falling out of the first round entirely.
"He'll probably be a star early because he's going to be able to use his athleticism," Roethlisberger said of Manziel. "It's not about your first year, it's about year two, three and four. Can you sustain it? When defenses have figured you out, can you figure out a way to stay a step [ahead] of them?"
Roethlisberger said he thinks the University of Central Florida's Blake Bortles will end up as the best quarterback in this draft class.
Bortles has been compared to Roethlisberger by ESPN NFL analyst Jon Gruden because of his size. The two also share an agent and both went to schools that didn't play in power conferences because they were overlooked in high school.
"I think he's going to be the best one in the group because I see something in his competitiveness, his leadership and I think he's got a lot of upside," Roethlisberger said of Bortles. "If you're a guy that comes out of a smaller school I think you should have a chip on your shoulder. It helps motivate you."