Sullinger averaged 6-6 w/ an injured back as the 5th option on offense.
The Celtics dynamic this year will cause Sullinger to be featured in the offense. When he is, he'll be able to showcase his ability and have the stats to match.
I expect Sullinger to eventually be just as good or better than Monroe; the per 36 and advanced metrics from their rookie seasons are considerably similar.
Trading Sullinger would depend on how successful they are as a tandem and Kelly Olynyk's ability (because you'd be choosing him over JS). In that case, you'd try to get the player that fits best with Olynyk which would probably be someone that could help him on defense, in this case, Drummond. Offensively, although Monroe has a polished offensive game, I think Sullinger's high BB IQ, high-low versatility, and passing ability will mesh well with Olynyk's ability. As long as they can be average defensively, they are going to destroy teams on offense and be a successful tandem.