According to Yakyubaka.com: "The Tampa Bay Rays and Boston Red Sox are mentioned as teams that might have an interest in the thirty-one-year-old infielder." He also will reportedly take a minor league contract from a team, which makes him attractive. He is a left-handed bat, right-handed glove and his OPS was .055 better than league average in 2012. His OPS was .085 better than league average in 2010 before the change in ball in the NPB. Compare this to flops in America Munenori Kawasaki (.020 worse than average in 2011 after the ball change and .026 better than league average in the last year of the old ball, and .056 worse than league average in 2009) and Tsuyoshi Nishioka (who averaged a .8034 OPS in his full seasons in Japan, which was about .060 better than league average over that time. So Tanaka is somewhat similar to Nishioka statistically.)
Here is a video of Tanaka's swing:
Here is Nishioka's swing from the right side:
So let's compare the swings:
Nishioka has a very dramatic leg kick in which he is standing on just one foot for a pretty long period of time (comparatively). Tanaka does have a leg kick where he brings his foot back, but it is much less dramatic. I think his gets off the ground less. They both have moving parts, but are both calm in the box considering. Nishioka's swing looks nice and flat while Tanaka's swing has more of an uppercut. Honestly, it is pretty easy to see what the Twins saw in Nishioka when they signed him. Tanaka's finish is much more violent as you can see he basically spins out of it, while Nishioka's is so much more controlled. So was plate discipline the issue? In the states, he walked just 6.3% of the time, but saw nearly 4 pitches a plate appearance (3.97) and swung at less pitches than league average. He made more contact than average, and only whiffed at about a league average rate. This was what he was good at with in Japan as well, as he was third in walks, 2nd in OBP, and first in batting average in the Pacific League in his last year of Japan.
So was Nishioka's problem power? That does seem to be the difference in swing between him and Tanaka. If you just watch the swings, you can image Nishioka is a more efficient hitter, but Tanaka gets more power. In the Majors, Nishioka's average batted ball was 243.776 feet, which is well below average (somewhere between 250-260 is where most hitters seem to sit). However, he didn't completely lack power in Japan, as he was 9th in the league in slugging. Of course, most of that slugging is held up by the batting average. In Japan, his best ISO was .167, which isn't bad (it would have been between Carlos Santana and Ian Kinsler in the states), but it obviously didn't translate to any power in the Majors (.021 ISO).
I am not a psychologist nor do I know anything about psychology that you can't learn in an introduction to psychology class (or 30 to 40 minutes of reading of good internet articles), so it does no good for me to speculate whether or not he "choked" or couldn't handle to pressure of either the MLB or changing countries. I am sure there are plenty of examples in business or professions where a very good professional in one country goes to another and struggles to do well and adapt. I don't know if that was the reason for Nishioka's struggles (as he was a disaster defensively, especially in the 2nd year, as well), or whether it was the lack of power.
If it is the lack of power, Tanaka may be in trouble. League average ISO was .133 in the Pacific League in the NPB in 2010 and then .096 in 2012. In those years, Tanaka's ISOs were .082 and .063 respectively. So if Nishioka's problem was power, than Tanaka doesn't look like a MLB player. Tanaka doesn't seem to have the same discipline and OBP skills that Nishioka had either, at least statistically. Before the recent reduction in stolen bases, it did appear that Tanaka had better speed and baserunning skills than Nishioka did, but I don't know that you can count on that anymore.
According to range factor, Tanaka is a excellent fielder. From limited video I could find, he seemed to have very nice range at 2nd base. The concerning thing is that 2nd base is all that Tanaka has really played in Japan. Could he play shortstop in order to be a true utility player? I would think a lot of that depends on his arm, something I didn't really get a good look at.
Rated really high speed wise, 93 according to Baseball Cube. With that said, his stolen base production and efficiency has really dropped off over the last couple of years.
I wrote a little bit about Takahasi Toritani statistically here. Here is some video of him making plays at shortstop:
It is pretty clear that he has the range and athleticism to play a good shortstop, but I do wonder about the arm, as many of the throws are pretty weak in the video. This may rule him out of playing 3rd base in a utility role, but it appears he has the ability to play an average to above average MLB shortstop.
I obviously prefer good defensive shorstops over good defensive 2nd baseman, but I really don't like Toritani's swing, as he is completely off balance:
In other videos I saw, he would completely fall down when he missed. This kind of violent swing is okay for Oscar Taveras, but not okay for a middle infielder. Before the ball change, he hit 20 homers one year and 19 the next, and slugged in the upper .400s. Since then, his slugging has dropped, but his OBP has actually improved (perhaps suggesting that he has made an adjustment). He doesn't steal a lot of bases, but he is efficient in doing so.
I don't think either are real starters (though they could probably hold their own). In a good world, they are utility players, with Toritani getting a boost because of his clear ability to play short, but Tanaka giving you other skills. These type of players coming from Japan seem to be undervalued and could help a team in a utility non guaranteed roster spot role.