Pittsburgh Steelers quarterback Ben Roethlisberger has admitted that he is unsure whether Johnny Manziel will have as big an impact in the NFL as he had in college football.

Manziel's credentials have been widely scrutinised in the build-up to this month's Draft, with the former Texas A&M Aggies quarterback polarising opinions amongst fans and pundits alike.

Roethlisberger acknowledged that the 21-year-old could do "special" things on the field, but questioned whether he would be able to maintain his success once defences wised up to his play.

"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 told 93.7 The Fan.

"The guys in the NFL are all bigger, stronger and faster. He'll probably be a star early because he's going to be able to use his athleticism.

"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?"

Manziel has been predicted to be chosen with the first overall pick by some experts.