If I recall, the snippet was that this would be the largest contract ever to a Japanese player. Largest. Let that word sink in. It says nothing about annual value. A 6 yr, $54M deal is the "largest" ever. Another case of the media getting a buzz word in there. Based on the way it was reported, we know nothing about the annual value of the contract.
I think we've allowed something to slip through the cracks. Is Boras really willing to risk him going back to Japan? I think the answer is yes if he's certain he can keep Matsuzaka as a client. But I don't think that is a certainty. Matsuzaka is a hero, as Boras mentioned, over there. If this falls through, I doubt Matsuzaka will get much blame publically. Boras will be the fall guy - it is his job to get the deal done. The Sox will get blamed too, but I think there's a strong chance Boras loses Matsuzaka as a client if this doesn't get done. Then he gets nothing when Matsuzaka becomes a FA. I think he takes the sure thing (money now).