Nathan Ahle/Triple 7
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I think this is an incorrect generalization about SP that come over from Japan. I agree that you should wait a few seasons to see if he is the real deal, but don't you think that applies to all up and coming pitchers with hype behind them. I mean it is pretty difficult for all SP to sustain their early success, not to mention any sort of prolonged success. This is not something that is unique to players just coming over from Japan.
Also, there really isn't all that much data on SP coming over from Japan, because there really has not been that many come over to gather many meaningful statistics. Especially when you compare it to data of SP coming up from farm systems and other professional leagues. I mean I feel like I can say with confidence that the success rate of SP coming from Japan's league is higher than those coming up from the minors. Although, that may not be a fair comparison b/c many consider the Japanese leagues a little more advanced than say AAA. I bet even if you compared to their numbers to other SP coming in from other international leagues or even top SP prospects after they have settled in- you would likely see a lot of simulates in raw performance averages.
I guess the point I am really trying to make is its pretty tough to stay constantly good over a sustained period for any MLB SP, not just ones that come over from Japan's leagues. There is only a select few that can really do this, no matter what league they come from. I think it has a lot more with a special type of talent and knowledge of the game than it does what league you in come from.
I think that view really comes from what happened with Dice-K. He had a decent first year and a really good second year, but after that he dropped off the map. Obviously Darvish and Tanaka are different pitchers entirely but people still had alot of apprehension about Darvish during his first year and now its happening with Tanaka.