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What does Sean Doolittle bring to Nationals’ bullpen?

Sean Doolittle relies on his fastball as much as nearly any reliever in baseball. (Gary A. Vasquez-USA Today Sports)

PHOENIX — Sean Doolittle‘s Nationals debut, which came about seven hours after his arrival in the Washington clubhouse Tuesday, was so eventful that it hardly allowed for much reflection. Even if he doesn’t close most games for the Nationals, he seems certain to factor prominently. What exactly does he bring to those late-inning duties?

Well, he mostly brings a fastball, a lively one that averages 94.4 mph. And he brings it again, and again — and again. Among relievers who have thrown at least 20 innings this season, Doolittle has used his fastball fifth most, 88.6 percent of the time. Ryan Madson, his teammate with the A’s and now Nationals,  compared that fastball to the one thrown by retired closer Billy Wagner, another fastball-heavy lefty.

“So when you’ve got something like that,” Madson said after Doolittle’s adventurous save Tuesday, “hopefully he’s just going to go back to that and rely on that.”

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Doolittle does not get himself in trouble often. Among that same group of relievers who have thrown at least 20 innings, Doolittle owns the sixth-lowest walks and hits per inning pitched (WHIP). His leadoff walk Tuesday night was more than a rarity; it was entirely out of character. He has walked 3.6 percent of the batters he has faced this season, tied for fifth-lowest in the majors, a stat that seems to correlate with late-inning excellence given that the only pitchers with lower rates are Kenley Jansen, Addison Reed, Roberto Osuna, and Mark Melancon.

But all those statistics are prefaced by “among relievers who have thrown at least 20 innings” this season because Doolittle has not thrown enough innings to qualify for regular leader boards. The 30-year-old missed part of this season because of left shoulder trouble, and has not thrown more than 60 innings in a season since 2014. Along with that fastball, Doolittle brings well-documented injury risk.

Doolittle explains that as a by-product of his unique road to the big leagues. The Athletics drafted him in the first round of the 2007 draft — as a position player. The first baseman/outfielder was the 2007 ACC player of the year at the University of Virginia, where he led the Cavaliers in RBI and tied a season records for walks.

“I protected [Ryan Zimmerman] in the lineup at Virginia,” Doolittle said. “I think he’s got better protection now.”

Doolittle worked through the Athletics’ minor league system as a position player, and hit 22 homers between High-A and Class AA in 2008. Then knee surgeries forced him to miss most of the 2009 and all of the 2010 season. When he returned, the Athletics converted him to a pitcher, at which point he did not have as much professional coaching or repetition as more polished minor leaguers. He didn’t seem to need it; he was in the majors by 2012. But Doolittle thinks he has honed his mechanics to preserve his health better now — an ongoing process that extended to this year’s injury and the rehab that followed.

“Changed some stuff mechanically, and it really took some stress and pressure off the front of my shoulder. After maybe my third or fourth rehab outing [in June], I felt back,” Doolittle said. “I’ve been really happy about how I’ve been feeling, how things have gone since I made that adjustment. Hopefully keep it going.”

The Nationals certainly hope he can keep it going, too. Doolittle’s numbers against lefties this year were stunning — at the time of the deal, they were 0 for 23 with 12 strikeouts against him — but his career splits show he hardly allows righties comfortable at-bats, either. Lefties are hitting .182 against Doolittle in his career. Righties are hitting .212. This season, lefties are hitting .039 and righties .218.

Given those splits, Doolittle fits well as a closer, one Manager Dusty Baker can use without worrying about matchups, though he has indicated early on that he will involve matchups in decisions about who pitches the ninth between Madson and Doolittle. Doolittle’s durability also will be a factor, Baker said.

“I’ve been told that you have to watch Doolittle’s arm because when he hurts it, he usually goes on the DL and is down for a while. So right now I don’t know,” Baker said. “I’ve also been told that Doolittle gets left-handers out exceptionally well, but you have to be aware of a fastball-hitting right-hander. But both of them get right- and left-handers out. … It was a probably a lot easier for Melvin to use them as he wanted to in Oakland because they weren’t in very many games, so [it was] easier to use them in games when it was meaningful.”

In other words, Melvin’s team had fewer leads to protect, so he could avoid overtaxing either reliever. Baker’s needs in that regard will be greater, so he will manage Doolittle and Madson much as he did Shawn Kelley when he was healthy: carefully.