Masahiro Tanaka is likely to draw a yearly salary of at least $17 million when all is said and done according to Bob Nightengale:
Team officials involved in Tanaka sweepstakes say the Japanese star will cost a minimum of $17 million a year— Bob Nightengale (@BNightengale) December 27, 2013
This, presumably, would put the Red Sox out of contention for the star Japanese pitcher. While the team hasn't verbally committed to the Collective Bargaining Tax threshold of $189 million as their effective cap, that's been the reality for a few years now.
With that in mind, there's just not enough room for the Red Sox to fit such a massive contract into 2014's payroll. Had Tanaka's posting been pushed into the next offseason, when the Red Sox will be offloading a bunch of money, they might have been involved. Even then, though, the Red Sox have an abundance of young pitching prospects ready to contribute to the major league team in the near future and might not be interested in gambling on a pitcher as unproven as Tanaka rather than simply bringing back free-agent-to-be Jon Lester.
The question now for the Red Sox will be whether he shows up elsewhere in the division. The Yankees are being pulled in two directions, and could well forsake their own $189 million goal if it means improving their chances for returning to the playoffs. They'll face stiff competition from elsewhere, however, and the Red Sox should probably hope Tanaka makes his way to the National League, or proves a massive disappointment if he doesn't.