Oh stop with the "ready-made excuse". You have no way of knowing for sure why Shields signed with the Padres, or knowing for sure what other kinds of offers he received. It's not making excuses anyway. It's trying to understand the rationale behind a decision, or in this case, a non-decision.
As I posted before, I thought the Sox should have been in on Shields at that price. But since they weren't, I'm not going to bash the FO, but rather try to understand why they weren't interested. They do have a good rationale behind their decision, whether you agree with it or not.
Also, Shields having a big season in San Diego would not necessarily translate to Shields having a big season in Boston.