Patrick Corbin needs to locate his fastball better in order for his dominant slider to work. (Gregory Bull/AP)

CHICAGO — The diagnoses for Patrick Corbin’s rough stretch have been a bit off, but it’s hard to blame Manager Dave Martinez and shortstop Trea Turner, given what they saw.

They both noted, on separate occasions, that Corbin isn’t getting opponents to chase his slider like he normally does. Martinez made that observation May 31, after a patient approach from the Cincinnati Reds led Corbin to pound the strike zone to climb back in counts. The left-hander, one of the Washington Nationals’ cornerstone starters, gave up eight runs (six of them earned) in 2⅔ innings, his worst outing of the year. Turner’s assessment came last Thursday in San Diego, after the final two hitters Corbin faced watched the lefty pound sliders into the dirt with no success. He gave up five runs in five innings and walked a season-high five batters.

And while Corbin agrees with Martinez and Turner, noting everything has to be better right now, he sees a bigger issue.

“Everyone wants to talk about my slider, and how effective it is, but it really can’t be without fastball command,” Corbin said in San Diego, a day after coughing up a four-run lead in a loss to the Padres. “It starts with my fastball, even if the slider is the pitch I want to get to. I’m just not locating right now.”

Corbin paused and rubbed his neck, clearly a bit frustrated with his recent results.

“I don’t know, I feel good overall,” he added. “I’m close to getting it all back to normal.”

Normal, for Corbin is being one of the three anchors of a rotation that makes Washington tick. Before that start against the Reds, when his pitches were smacked and sprayed all over the field, Corbin was coming off a complete-game shutout of the Miami Marlins. His ERA was down to 2.85 and his record improved to 5-2. But then he gave up 16 hits and nine earned runs in back-to-back losses. The Nationals are on a 12-4 run, inching within four games of .500 at 31-35, and half of those recent defeats can be pinned to Corbin’s left arm.

He put them in an eight-run hole before they could blink against the Reds. He let the Padres come all the way back to halt a four-game winning streak. The Nationals aren’t expecting him to be perfect, $140 million contract or otherwise, but they do need him much better if this slight turnaround is to become something more. They are on another three-game spurt, having just routed the White Sox, and Corbin gets a chance to rebound Tuesday in the finale of a two-game set..

It’s hard not to key on Corbin’s slider, when he’s either succeeding or stumbling. Yet he’s looking elsewhere.

“The foundation is spotting my fastball, early in counts and especially if I fall behind,” Corbin said. “That’s where it all begins."

This was his final answer as he was pestered with theories about his recent downturn. He listened to them patiently, even smiling at a few, and offered measured responses in the corner of a cramped visitors clubhouse.

He threw 107 or more pitches in four of his five starts before the meltdown against the Reds, including 116 in that shutout of the Marlins. So, how about fatigue? “No, no. My arm feels good.”

He threw his four-seam fastball fewer than he has all season in the last two starts, basically trimming his arsenal down to a slider and sinker, which is also referred to as a two-seam fastball. So has he become predictable? “No, I throw four-seamers more to lefties and recently have faced righty-heavy lineups.”

And how about his manager and teammates’ theories about teams laying off his slider, forcing him to come into the zone with something else? “It makes sense. But the slider will work better in those cases if the sinker does.”

His wavering fastball command affected him in different ways. He gave up 11 hits against the Reds because he was constantly falling behind. He threw a first-pitch ball in seven of the 11 at-bats (five of them fastballs) that ended with hits. The four other hits came on the first-pitch of at-bats, and three of those were fastballs.

Against the Padres, he struggled to throw his sinker for strikes and wound up walking five batters, crowding the bases and spiking his pitch count. The slider was ineffective then, too — especially against Wil Myers and Hunter Renfroe in his final inning — but that was symptomatic of not setting it up well.

“When he locates his fastball, he allows his other pitches to become a threat,” said a scout who watched Corbin’s start in San Diego. “When he doesn’t, [hitters] can eliminate his slider and hunt his fastball mistakes.”

Corbin throws his slider more than 30 percent of the time, according to the pitching analytics website Brooks Baseball, similar usage to his four-seam fastball and sinker. And one of the reasons the slider is so good, and considered one of the better out pitches in baseball, is because it looks like his fastballs until its path takes a sharp, sweeping turn from left to right. That happens about halfway to home plate, once hitters have already started their swing and are expecting something straighter and 6 to 8 mph faster. Then they are often left shaking their heads.

But the slider becomes much easier to resist if his fastball is ineffective (like against the Reds) or erratic (like against the Padres). Corbin only sparingly throws the slider when down in counts, especially against righties. His dominant pitch can’t function if the rest of his limited options are lagging behind. And so Tuesday against the White Sox, with the Nationals surging, is an opportunity to steady himself before inconsistency becomes a troubling trend.

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