Guthrie had a 4.00 ERA during his time in the AL East.
Even if the Red Sox spend 100 million for a guy like Greinke/Sanchez or bid on Haren, they may still end up as worse players because of the transition from the NL or from the AL West.
Put into perspective, Greinke has a 3.77 career ERA, while Shaun Marcum has a 3.76 career ERA. His health issues are concerning, but once again, AL East experience, and a bargain.