Nelson was optioned to Triple-A Scranton/Wilkes-Barre on Saturday, Randy Miller of NJ Advance Media reports.
The flamethrowing right-hander struggled with his control this spring, allowing four walks in five innings, though he did hold opponents scoreless in his three appearances. He'll begin the season in the minors, but Nelson has a chance to contribute to the big-league club this season if he can find a way to cut down his walk rate.