Baseball Prospectus compiled all the data from the past 10 years, and home teams that had a save situation entering the ninth inning -- leading by three runs or fewer -- converted 87 percent of those saves. Road teams converted 86.1 percent of their ninth-inning save situations.In other words, whether it's Nathan or anybody else standing on the mound, the Twins have less than a 14 percent chance of blowing that save."
It actually doesn't mean that. It means that is the average, but some pitchers are below average - perhaps a long way below average.
The other problem is, as most people know, the real difference is not how well pitchers do with a three run lead. It takes a real meltdown to blow one of those and those are pretty rare for any quality reliever. If you dilute the sample with a lot of sure things - you get a high percentage of success. But it tells you nothing about the relative ability of the closers, it just means you have masked it with a lot of situations where failure is basically random.
A closer who blows three or four more games than the next guy costs his team three or four wins. That is enough to decide a close pennant race. And if the alternative to Nathan pitches like Dave Stevens, it will be more than three or four games the Twins lose as a result.