Brian Cashman made some carefully worded and much publicized statements about Masahiro Tanaka earlier this week. Among other things, he characterized Tanaka's upside as a "#3 starter." One can reasonably conclude that Cashman's declarations were designed to temper expectations, relieve pressure, and perhaps even implicitly acknowledge the potential competitive advantage of pitching in the middle of the rotation.
Cashman also pointed to several oft-overlooked factors which make the transition from the Nippon League to MLB challenging. As has been widely observed due to the publicity surrounding Tanaka's exceptionally long postseason outings in 2013, Japanese starters throw more pitches per outing, but they also pitch less often. The strike zone is called substantially larger in Nippon, so more contact is encouraged, lineups have fewer power hitters, and a greater premium is placed on defensive ability. Even the ball is different, slightly smaller.
It seems logical that Cashman and Yankees fans expect some growing pains during Tanaka's rookie season (as you would with any rookie). Previous high-profile Japanese starting pitchers have almost uniformly been adversely effected by their transition to MLB. Here is a graph of average monthly Game Scores (a Bill James invention featured at Baseball-Reference.com) for Yu Darvish, Hiroki Kuroda, Daisuke Matsuzaka, Hisashi Iwakuma, and Hideo Nomo during their first seasons in America.
As you can see, performance is all over the map, even for pitchers who had relatively successful opening campaigns (only Nomo had a truly great year). Every one of these pitchers had an average Game Score under 60 during their first month as a starter in the United States. Several sunk even lower at various points during the season, but every pitcher also posted a 12-15 point jump during an equal stretch later in the year. It may be interesting to note that three pitchers - Darvish, Dice-K, and Nomo - followed relatively similar patterns through August, after which they diverged dramatically for the final month. One might surmise the each responded differently to another distinct difference between NPB and MLB: the longer season.
Compare the above graph to the following one for their sophomore seasons:
Not only does the whole group shift up 5-10 points, the lines get dramatically straighter as we have less month-to-month variance. Nobody peaks as high as Nomo did during his famous summer of '95, but nobody bottoms out like Dice-K and Kuroda did during their rookie seasons either. Even Darvish, who had a fairly solid rookie season and is generally pointed to (probably unfairly) as the best case scenario for Tanaka, spent three months hovering around the 50 mark (which is pretty bad). He also got nearly to 70 in September (which is excellent). During his sophomore season, on the other hand, he tallied an average Game Score between 58 and 63 in four out of six months and the difference between his peak (September, again) and valley (May) was less than 12 points.
I don't want to encourage overanalyzing this data, as each player's response to the transition could be heavily influenced by a variety of unquantifiable factors (health, temperament, culture shock, language acquisition, etc.). However, one thing the Yankees may want to control is how much Tanaka pitches during his first season. Every team except Boston found a way of limiting starts during the first year. It's probably not a coincidence the Dice-K was the only pitcher who absolutely crashed and burned in September. He was also terrible throughout the playoffs in 2007 (though the BoSox managed to survive despite him). The Mariners avoided such fatigue by sending Iwakuma to the bullpen for his first three months. He was pretty good down the stretch in 2012 and was then a breakout star in 2013. But because Tanaka is a more high-profile signing and fans are itching to see what he can do, one imagines the Yankees would be reluctant to bring him along so slowly. In this case, the Darvish comparison may be worthwhile. The Rangers used the All-Star Break to give Yu a prolonged rest, skipping his turn before the Break and slotting him into the middle of the rotation when games resumed. As a result, he only officially missed only one start, but he got two full weeks off. He was stellar during the pennant race and pitched well in his one postseason start.
What's most important to remember is that if and when Tanaka falters during the coming season, it should not be viewed as proof he is the next Kei Igawa. Of the five pitchers in the above charts, only Nomo posted his best season during his rookie year and even that was largely due to a two month stretch during which he was unsustainably out of his mind. He was much more consistent - with lower highs, but also higher lows - in later years. Hopefully, we will get glimpses of Tanaka's potential by the end of 2014, but his true value, whether as a #3 starter or something better, won't yet be certain.