James Young has the same big problem he had in college: He plays no defense. The Celtics fugured he had the size and athleticism to play average to better D, but it hasn't materialized.
There is no point in letting him play and adjust on offense while being a matador on the other end when you're trying to win (I would argue that they should be tanking anyway but I digress).
Lots of players leave early because they know they'll get taken early. The decision isn't solely about whether you feel ready or not to play at the next level. College, IMO, is a way better place to develop than the D League or tucked completely away on an NBA bench like Lamb was.
If you're eager to just go pro based on your ceiling and are confident in yourself, Kentucky easily could be your best option. But for a guy like Gabriel who likely will need time to develop his skill level and body, staying in school will likely help him in the long run, even if it is only a two year college stint. Duke, Maryland, UConn, or even Providence with Dunn to play with as a freshman would be good choices for that. Kentucky isn't the school for players in need of development though IMO; it's a place for the guys that wouldn't have gone to college a decade ago.