Demetrius is fast and athletic. He's good with the ball, a willing defender, and an unselfish ball player. His baseline is backup point guard; and with experience, he can be a starter on some teams.
As with ll NBA guards, shooting will determine how much Jackson plays. Just about every NBA starting guard is a good shooter, and many who come off the bench can shoot but do little else.
Boston is a tough situation for Jackson. Isaiah Thomas is the entrenched starting point guard. Terry Rozier is the #2 point guard; and because the Celtics spent a first round draft pick on him, he will get every opportunity to succeed. The good news for DJ is that Rozier has shot like (poop) his first two seasons in Boston - 35% overall and 30% from three point range. The bad news for DJ is that Rozier's percentages improved from season 1 to season 2, so improvement again in season 3 will not leave a spot in the rotation.
There are other teams that can use a guy with Jackson's skills, so I think he'll get a chance; and given a chance, I believe he will succeed. The aforementioned Isaiah Thomas is a second round draft pick who has become an all-star guard on a good team. Thomas spent three years in Sacramento, part of a season in Phoenix, and a season and a half in Boston before becoming the Celts' full time starter. Like Thomas, Jackson needs to get on the right team.