Free agent closer Joe Nathan, who's coming off an outstanding 2013 season with the Rangers, is likely headed for a multi-year deal, even though he'll turn 39 later this month. In some senses, that's understandable, as Nathan, when healthy, has pitched at a high level for almost his entire career and is coming off a 2013 campaign in which he posted a 1.39 ERA across 64 2/3 innings.
On the other, perhaps more important hand, teams who think they may be getting that kind of output -- i.e., a pitcher capable of sub-1.50 ERAs -- and are prepared to pay for that kind of output may be misguided. That's because underlying numbers suggest that Nathan won't be so dominant in 2014 and beyond.
For instance, if you look at Nathan's 2013 advanced stat line over at FanGraphs, you'll find he's a bit of an outlier in terms of his batting average on balls in play (BABIP), percentage of base-runners stranded (LOB%) and percentage of home runs per fly balls (HR/FB). Those, in fact, are the three primary indicators you eyeball to determine if a pitcher was lucky or unlucky over a given span of time.
In Nathan's case, all three suggest he benefitted from good fortune in 2013. His BABIP checked in at .224, which is well below his career mark of .253 and well, well below the expected range for all pitchers of .290-.300. In terms of LOB%, Nathan's 2013 mark of 87.2 percent is well above his career rate of 79.4 percent (which, in turn, is well above the expected rate of 72 percent). As for his HR/FB, Nathan's percentage of 3.0 -- versus a career percentage of 7.0 -- almost certainly isn't sustainable. Summing it all up is a FanGraphs metric called xFIP, which corrects for all these factors and suggests that Nathan's 2013 ERA should have been a more mortal 3.27. On another level, Nathan, per Brooks Baseball, showed some velocity loss relative to 2012 levels.
None of this is to impugn Nathan's work this past year. Those runs were prevented, and that ERA of 1.39 was factually accomplished. This, rather, is about what can be expected from Nathan moving forward and as he heads toward age 40. More to the point, it's about what a team should pay Nathan in light of those factors. Nathan's had an excellent career, but players, almost without exception, don't suddenly set a higher bar for themselves at age 38.
A number of contenders need bullpen certainty in the ninth inning (the Tigers, most notably), but they should know that the Joe Nathan of the future almost certainly won't be as dominant as the Joe Nathan of 2013.