If he's ready to make an impact now, then why has Tavarez been starting? You better believe it's going to take a ton for a proven veteran lefty (only 28, BTW), under contract for 4 years at under market value. Yes, that's top prospects...if you're not willing to pay, oh well...there are other teams that will (including, I think, the Yanks).
We have other pieces that can be traded. Javy Vazquez is signed for the next 3 years at market value, Dye can net some return, Thome, Crede will be traded next year. It's not like Buehrle is the only guy worth a s*** on this team. When you sign your best pitcher under market value, you don't trade him; you build around him.