Jarryd Hayne will prove to be a nightmare for opponents as he finds his feet in the game, according to the NFL's director of football development Matt Birk.

Citing Hayne's slick running skills and uncompromising toughness, Birk was left questioning how the 27-year-old can be stopped after watching him in the preseason.

"He's kind of a tough match-up if you will on the opposing team, because what kind of player can sit on Jarryd?" Birk told Sky Sports Radio on Monday.

"He's too big, he can overpower the smaller, faster defensive backs and he's probably too fast for some of the bigger, stronger line-backers.

"And that is key in our game of American football. It's about the match-up and Jarryd with his unique skill-set causes a problem for defences there."

The former Minnesota Vikings and Baltimore Ravens centre, who was twice named an All-Pro, believes Hayne will be eased into his offensive role as a running-back, and expects 49ers coach Jim Tomsula to first utilise the former rugby league full-back as a punt returner.

"I'm sure they will start slow with him because he still is new to the sport," he said.

"Because he is such a good athlete I would imagine he will be playing more of the phases, the kick-offs, the punts, the kick-off returns and the punt returns.

"Even though that is probably less fanfare, it is a very crucial part of our game: the kicking game.

"He will be getting plenty of time and I can imagine as the season goes on, offence is a little bit more complex and they will probably work him in there."

The 49ers will open their campaign when they host the Vikings in Monday Night Football (Tuesday AEST) action on September 14.

AAP