The important question is, what is a premium. I think that he should at least net us a future first where other posters believe he is only worth a trade exception and a second
A trade exception doesn't matter too much because the Celtics would have to renounce it this summer if the team used cap space to sign free agents.
One example of a trade that involved a rental of a veteran for the last months of the season was last year when Houston traded Shane Battier and Ishmael Smith for DeMarre Carroll, Hashem Thabeet, and a first-round pick.
Thabeet had a guaranteed contract for 2011-2012, so I think the market for a rental is set at a non-lottery first-round pick if you take back salary committed to next season from a player unlikely to contribute. So, if you don't take on that salary, then it seems logical to me that the pick gets down-graded to a second-round pick.
For example, if you have visions of getting a first-round pick in a deal with Chicago, I don't think the Bulls should agree unless they get to dump Rip Hamilton's contract (two more years at $5m per, the last partially guaranteed for $1m). And if you get a first, it would certainly be the Bulls' own pick and not the protected Bobcats pick.