This is not a CD-informed guy talking but more a random lurker talking (especially because I'm not sure if I can talk about year-to-year save feature at all yet), but my guess is that those MLB players are internally tagged as such, since you cannot edit them on MLB 14. Also, MLB players cannot be exported from roster, so the game must know they have real-life counterparts. Therefore when they carry over to MLB 15 such players should be identified easily by the game by the tags and some sort of player identifiers.
Now, I just checked on MLB 14 and I think Tanaka is one of those players added to rosters via the CAPS, in which case he may not be tagged as someone who has a real-life counterpart in the game... so you may be out of luck and the game might not replace him with real photo and such.
(However, given that Tanaka may be one of few bright spots for the 2015 Yankees, it's not impossible to imagine that the devs might go extra mile to make an exception for him... you never know. Since I am no Yankee fan, it is no big deal either way, so long as Bartolo Colon on the Mets remains fat in the game.
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