only reason to trade Grant is if some contender sees him as a vital piece to a deep playoff run and they offer you a 1st round pick and a young player with upside, something like that.
Grant is a cost controlled role player who knows his role, is lights out from outside, and usually knows where to be and how to execute plays, even if he's too short to make an impact on defense as often as you'd like.
For Grant? 
yeah, i mean ... Grant is a 1st round pick on a rookie contract, is a lights out spot up shooter with low usage, plays a position where it is difficult to find shooting + adequate defense.
so if the Celts are trading him, you would need to be getting back something more than just, say, a late 1st round pick. what's the point of using a late 1st round pick on a player, that player turns into a decent role player, then you trade him for a late 1st round pick before his cost even goes up?
I'm assuming a contender is not offering a higher quality 1st round pick e.g. late lottery / mid-1st. If a playoff contender could offer a pick likely to fall in the 10-15 range, then I think trading Grant straight up for a single pick would make sense. But if all you're getting back is a late 1st round pick, that seems like negative value to me. After all, most late 1st round picks don't turn into rotation players.
In short, if you're trading Grant, to me that only makes sense if you're getting a return that seems kind of exorbitant. Grant as a trade piece is the kind of commodity that teams in a position to win ought to overpay for .... cost controlled low usage role players who could really help a contender for multiple playoff runs.
If the other team doesn't make a really nice offer, you may as well just keep him. If they don't want to pay him when he's about to his RFA they can just trade him at that trade deadline for a lesser pick, or do a sign and trade when he hits RFA.