After today, Tyler Clippard now has 16 relief appearances and 23.2 innings pitched on the season. There are pitchers in the league with more relief appearances (the Mets' Fernando Nieve leads the league with a whopping 20), but no other pitcher has thrown as many relief innings. Of all the pitchers with 16 or more relief appearances this season, only Robinson Tejeda of the Royals and Matt Capps are averaging more than an inning per appearance. And with only two low-leverage appearances, Clippard is pitching in almost exclusively higher pressure situations.
Make no mistake, Clippard has gotten very good outcomes, making it hard to blame Riggleman for riding the hot hand. He's striking out 31% of batters faced (more than Neftali Feliz or Mariano Rivera, to cherry-pick two examples), which allows him to overcome his 54% flyball rate and 12% walk rate.
Of course, before we get too Clippard-crazy (too late, I know), we have to remember that he has been the beneficiary of some excellent luck, too. The .218 BABIP against has allowed him to keep his WHIP below 1.00, despite all those walks. And he's going to have to endure a pretty long stretch of beatings before his minuscule 3.9% home run-to-flyball rate regresses back to the league average (just ask Brad Lidge about his 3.9% HR/FB rate in 2008). His 3.90 xFIP is a better indication of his true performance level than his 0.76 ERA or 6 wins. And even with all that luck, he's actually allowed 10 of 18 inherited runners to score, a really pretty terrible 55.6% rate (league-average is 32%).
But the main point of this post isn't to dump on Clippard--it's to worry about him. Especially with Brian Bruney showing exactly why he was left off the Yankees' playoff roster for the first two rounds (and then used exactly once in the World Series, allowing 2 runs and recording one out), Clippard is crucial for the Nationals ongoing respectability. And right now, he's on pace for 123.1 innings. Obviously he's not going to throw that many innings, but you have to wonder whether even a month at this pace will take a toll.
Relief pitcher usage is a poorly researched area. Sample sizes are inherently really small, and relievers are so erratic from year to year anyway that it's hard to separate out the effects of usage from luck and skill. Still, we all watched Luis Ayala, Jon Rauch, Chad Cordero, and Saul Rivera fall apart racking up a ton of relief innings between 2005 and 2007.
Again, I don't blame Riggleman for riding the hot hand or trying to lock in every win he can while his team is playing well. The problem could solve itself if the team goes cold for a week or two. But pretty soon something's gotta give. If the team does keep producing leads to protect, either someone else needs to step up or Rizzo's going to need to make a move.