I think he'll be our fourth-string center this year, and he will play a bulk of the year in Maine.
The main reason is that Kanter, Theis, and Poirier are all likely better than him. Kanter should be self-evident. Theis is an experienced player whose knee will be in better shape than last year. Maybe Williams is better than Poirier, but I would guess Poirier is still more effective because of the experience factor.
Could some of those four guys play minutes at the 4? Probably not, if you ask me. I think it's equally or more likely that Stevens plays some lineups with none of them on the floor, and with Grant Williams or Semi Ojeyele as the biggest player on the court.
If he doesn't play much this year, that doesn't mean his career is over. I think the team will be content to see him continue to improve, with the possibility of being a contributor next year. It takes time for bigs to develop and I'm sure the team understands this.