This is a mistake by Sandy Alderson and the New York Mets.
After not pitching at all in the major leagues in 2010, Isringhausen has come back strong in 2011 to the tune of a 2.70 ERA and 7.9 K/9. It's hard to imagine his value being any higher than it is right now.
With the trades of Francisco Rodriguez and Carlos Beltran, the Mets are clearly sellers, so why not get a prospect back or two for a guy who is in demand and will be 39 in two months? The Mets got everything they wanted out of Isringhausen and more in 2011, so holding on to him wouldn't be prudent at this juncture to quote my inner George Bush Sr.
I think the Mets are missing the boat here.