He played pretty well as a rookie last year. I have no idea why he isn't playing this year. Not sure if that is a red flag or just the Knicks being the Knicks.
I wouldn't give up much for him, but he is certainly more deserving of a roster spot then someone like Nader.
We don't need Horford, Baynes, Theis, Monroe, AND Willy.
We've got plenty of traditional bigs already.
Generally speaking, I would say that the Celts should not invest any significant resources into acquiring a center unless that center is a star who can impact the game on both ends. The market for centers these days is such that you can probably always find a useful contributor for relatively little $$.
Baynes, Theis, and Monroe are good examples of several different avenues for this (NBA veteran, undrafted foreign FAs, buy-out market).