No he wouldn't be a backup exactly. He would be in a position battle. And given the outcome of the last 3 years (unfortunately last year affected by injury), and the pedigree of NK. He is wouldn't begin off season practice with the starters. And he'd be behind Mullins, and NK. and he ended this season behind Ross.
But Hurley will play the best players. Stewart still would've had the opportunity to win the position. But the odds weren't in his favor. Since he couldn't get a guarantee to start as a senior, since he knows that's not our culture and Hurley couldn't promise him that, he had to leave for a chance play more, which could improve his long term prospects for a basketball career.