Glenn is a very good 3rd/4th big, and I'd love the Celtics to sign him to a contract about Perk's current money. I don't think he would be a good starter, but he'll keep improving his shot and skills, so he could end being a 6th man averaging 15 and 7 (he's not there yet) in respectable minutes.
If he's willing to take that money the Celtics must resign/extend him long term, he'll never be a star, but he's solid, and that will be much needed in the Rondo Era.
If he wants more money, let him expire, I don't think there will be a good offer for him at the deadline. If a good trade (big if) happened, the Celtics will need to pull Sheed from retirement.