You take a look at the situation.
The Red Sox have a team constructed right now that could win a WS again. It really wouldnt even take too much luck for it to happen. They have an ace. They have a Japanese kid who certainly could be very good for a very long time. They have another future ace in Buchholz who isnt even in the trade discussion. You have a lineup that will again be top 5 in all of baseball in runs scored. You have possibly the best bullpen in the game. And, you have one of the better farm systems. Right now, if you could add the best pitcher in all of baseball to your staff without subtracting even one player from the roster that you started the season with, then why the hell not do it? What does it matter if Ellsbury and Lester are included. And then throw in 2 rookies. What does it matter? You add Santana and you have a 1-2 duo of epic proportions. If DiceK and Buchholz live up to expectations, you will have a 1-4 that could be aces on any other team, and all will be in their 20s. And you can do this without subtracting 1 player from your team that started your championship run. Not 1.
The Yankees OTOH, would need to acquire Santana just to get to where the sox are right now. The Yankees lack the shutdown ace, the sox have one. But we have 2 guys who certainly could be aces. What if you deal the wrong one? Then you are stuck repeating the mistakes of old and treading water against the sox, who no matter how the dealings end, will still have the better bullpen. This is why the yankees need caution.
But this is also why the sox should make this happen. If they want Ellsbury...so be it. Lester too...Add him to the pile. How about Lowrie and Masterson? Go ahead. None of those guys hurt you right now, and none will hurt you even 1/10th the amount that Santana will help. Allowing Santana to go to the yankees makes this a rivalry again, and most likely tips the scales in the yankees favor. Getting Santana makes this akin to the reverse of the late 90s rivalry. The yankees would be doormats to the sox, on their way to continued, long lasting, epic success. The only thing that current yankee fans can hold over you is the dynasty plug. 4 in 5 yrs! The sox have had 2 in 4yrs, but with Santana could get to 3,4,5, hell who knows how many. This is a dynasty building move without having to serious hurt the foundation of your team. This is why Theo has the duty to overpay so long as he doesnt have to deal away someone like Papelbon. This is why he should do it. Because without Santana, they still have a solid chance of repeating. With Santana, it will be like the 07 Pats, slaughtering teams on their way to the inevitable.