Repeat after me: The worst part about losing Joe Nathan for the season isn’t losing Joe Nathan for the season.
Nathan would have been worth ~2 wins this year. His past three years were 2.2, 2.1 and 1.9 so we can say that with a pretty high degree of certainty. His likely replacements, Jon Rauch or Matt Guerrier, are worth roughly 1 and .5 wins apiece. Rauch has had better strikeout numbers (except for last year) so it would probably be best to put him in the middle innings to give him more high leverage situations, then give Guerrier the opportunity to get 3 outs with nobody on base. Replacing Nathan with a lesser pitcher might cost them as much as a win (although I don’t think it would even be that high when you factor in Nathan’s likely decline anyways) so that is not the problem.
The problem is farther down the depth chart. Now all of a sudden every reliever gets bumped up a spot. Guerrier closes, Mijares and Rauch will get roughly the same innings leverage wise, but now Jesse Crain gets higher leveraged innings, etc. What happens when Clay Condrey comes in with 2 runners on and 1 out? You can see where problems arise from having to use your 7th reliever as your 6th reliever, and so on.
Another overlooked problem is this will certainly require the Twins to use 12 pitchers, and I can envision them using 13 for parts of the year. This means less opportunities for Alexi Casilla to run for Jason Kubel, or Jim Thome to pinch hit for Punto. Although Nathan himself isn’t irreplaceable, his impact on the team very well could be. The ramifications of him being out for the year probably won’t cost the team the division, but it could cost as many as four wins. And, given that the central has been close (much closer than I’ve predicted) in the past couple years, I guess it is possible that Joe Nathan going down could result in the Twins not winning the Central.
Which is why the Twins should go sign John Smoltz. Without a job yet he will likely come cheap, and he has closed before so he will be able to step right in and do so this season. His 6+ ERA last year was unsightly, but he still managed to strike out almost a batter per inning while keeping his walk rate around 2 per 9, in line with his career numbers. In the bullpen, Smoltz’s stuff can play up for the 20ish pitches he will throw and he will more than likely strikeout around a batter per inning. His low groundball rate won’t be as big of a problem when he gets to start his own inning either. His fastball was below average last year, but pitching out of the bullpen should fix some of that, and it will make his legendary slider, which was plenty effective last year, even more effective. Signing Smoltz could save the season. No pun intended.