We have eight Japanese born pitchers with at least 50 starts in the major-leagues, and I've excluded Tomo Ohka, who threw less than 60 innings in the top flight there, before coming to Boston. Not a great sample-size, but we have to work with what we've got. For each, the table below shows their stats over the final three seasons in Japan, their age during their first season in the majors, and the same stats during their first three seasons here (or their entire time, whichever is less). I've also incluTanaka's Japanese stats for comparison.
[Quick aside: as in the US, one Japanese league uses the DH, one doesn't, so moving to a different format league can have an impact. But not getting into that here!] There is, obviously, a fair degree of variation. All the pitchers saw their ERAs increase, as you'd expect, but the increase varied enormously. Nomo only went up by .05, while Irabu increased by over two runs. Darvish's ERA more than doubled, but he was coming in from an almost insanely low amount. However, we should bear in mind, so is Tanaka. Overall, the percentage increase for ERAs by each pitcher was a little more than half (150.2%), with an increase of 1.20 runs over the Japanese figure in their first three years.
Looking at the peripherals, K-rates tended to drop somewhat (91.6% of the Japanese figure), while walk-rates increased by about a third (133.5%). But it seems to be the home-runs which appear largely responsible for the increase in ERA. Japanese pitchers seeing their pitches leaving the park at an average of more than double the rate (202.7%) they did at home. It's also perhaps worth noting the large variations in age when making the transition, from 25 for Darvish and Tanaka to 33 for Yoshii and Kuroda. This is important, because it appears the peak age for MLB pitchers is 27, so Tanaka still has his best years ahead of him.
Certainly, a quick glance at the numbers would suggest Darvish as a good comparison for Tanaka, with similarly insane ERA, identical home-run rate, and both strike out more than average, while walking fewer. In October, one scout was quoted as saying, “He is better than Darvish because he is a strike thrower. Overall, Darvish’s stuff might be a little bit better, but this guy knows how to pitch," with the same piece going on to say, "Some evaluators believe Tanaka could be the best pitcher ever to come from Japan." However, comparing Darvish's stats to Tanaka is tough, because the latter's last three seasons coincided with a new ball in NPB, which severely decreased scoring.
Others think Kuroda is a better comparison, led by Jeff Sullivan who says of the pair, "Both have fastballs around the low 90s. Both throw a lot of sliders, both are known for their command, and most importantly, both feature a frequent splitter." There have been concerns about Tanaka's fastball, Baseball America's scouting report warning it "gives some scouts pause because it comes in on a flat plane, making it more hittable than the velocity might suggest." However, the splitter is acknowledged as particularly filthy. Teammate Brandon Duckworth said of it, "he throws with the same arm speed so that it comes out just like a fastball. It's crazy because that thing just falls off the table."
Even those who don't think Tanaka is quite as good as Darvish are hardly dissing the man. Connor Jennings, whose site is dedicated to projecting Japanese players' performance in the majors, says, "While Tanaka has good strikeout stuff, and excellent control, I don’t think he will be quite as good as Darvish. However, he should easily be a front of the line pitcher... He is a safe bet to be around a 4 win pitcher, with potential for more. He has the potential to come in and be the ace of a staff." He pegs Tanaka for a 3.59 ERA, FIP of 3.03, with a K-rate of 8.9, walk rate of 2.9 and home-run rate at 0.7, which doesn't seem too implausible.
Of course, right now, we're still in a holding pattern, without any word as to whether Tanaka will be posted. There was some suggestion Christmas Eve and Christmas Day were likely ones for the announcement, but it's just gone past 1am on Christmas morning there, so we can cross the former off the list. But with the list of clubs interested now apparently led by the Diamondbacks, Cubs and Yankees, it might be nice to get up tomorrow morning and find, if not a Japanese pitcher under the tree, at least the ongoing possibility of one.