The British discus record holder took more snaps that any other defensive lineman during off-season practices with the Super Bowl runners-up.
The 21-year-old told the Sacramento Bee: "It's the hardest thing I've ever done, I'm under no illusions."
The 49ers' defensive coordinator Vic Fangio added: "He hasn't had anybody hit him yet. He hasn't had a double-team yet. He hasn't figured out if it's a run or pass, whether he should rush or play the run. So it'll probably be a very slow process with him much more than a normal rookie."
The Olympian announced his intentions to move to the NFL earlier this year, despite having never played American football.