Sean Doolittle has relied heavily on his fastball this season. (Jonathan Newton/The Washington Post)

When the Washington Nationals traded for Sean Doolittle last July, they didn’t receive a very good but injury-prone and aging reliever with limited closer experience from the Oakland Athletics. They ended up with an evolving veteran with untapped potential poised to make a leap.

Less than a year later, that progression has yielded Doolittle’s second all-star nod in his first full season as a closer, and he has fit in seamlessly in the clubhouse and city. He won’t pitch in Tuesday’s All-Star Game at Nationals Park because of a foot injury that unexpectedly surfaced in the first week of July. Still, he is a better pitcher enjoying a career season. It is not by accident.

“When you believe in that process, when you trust it, you can get that much more out of it,” the 31-year-old said. “I feel good about where it’s helped me get to.”

Doolittle’s development has been two-pronged, involving both his mental well-being and arm maintenance. The physical portion began in his final months in Oakland, when the left-hander commenced a new strengthening program focused on fortifying a shoulder that had landed him on the disabled list four times over his final three seasons there.

The regimen advanced as soon as he arrived in Washington. Doolittle was open with the Nationals’ training staff when he joined the club. He felt healthy at the time, but his injury history was impossible to ignore. He wanted to try something different to avoid another repeat. His new program incorporates heavier weights with fewer reps. Bodybuilding competitions aren’t in his future, but Doolittle said his shoulder has felt stronger since he took up the routine.

“It stinks because you’re using like an eight-pound weight,” Doolittle said. “And you’re like shaking because it’s all those small stabilizer muscles in your shoulder, and you’re all like, ‘Aghhhh.’ So it doesn’t show as much as I would like, but I’ll take it. My shoulder’s getting swole. So knock on wood.”

The program’s effectiveness was immediately evident in the workload Doolittle assumed in Washington. Before the trade, he logged 21⅓ innings across 23 appearances. After it, the burden amplified to 30 innings in 30 games with the Nationals. He tallied a 2.40 ERA with Washington. Most importantly, he held up. This season, he pitched to a 1.45 ERA and compiled 49 strikeouts against three walks and a hit batter in 37 1/ innings before he felt pain in his left foot jogging out to the bullpen July 6 against Miami.

He pitched that night, tossing a perfect ninth inning to earn the win, but the discomfort didn’t subside. A few days later, he was diagnosed with a pinched nerve between his big and second toes. He was placed on the 10-day disabled list Tuesday and isn’t eligible to return until after the all-star break. Whether he is activated immediately is unclear until he tests out the foot again. He and the Nationals will proceed with caution given his arm injury history.

When healthy, Doolittle has succeeded by relying on his fastball, a pitch with a rare spin efficiency that appears to jump on hitters, 88.9 percent of the time. Only two relievers have thrown a higher percentage of fastballs.

Doolittle has allowed a run in just four of his 35 appearances. He has secured more than three outs in four of the games and didn’t allow a run in any of them.

“He just goes after you, and he knows how to pitch,” Manager Dave Martinez said. “Now he’s got three pitches he feels comfortable throwing. Mainly he’s a fastball pitcher, and he knows that. So he attacks the hitters.”

In Oakland, the shoulder injuries and occasional poor performance weighed on Doolittle, chipping away at his confidence. He would build up each outing to be a do-or-die situation. The pressure became unhealthy. Innings would unravel, and the stress would mount. Everything would speed up.

This year is different, he said, because he has worked with Mark Campbell, the Nationals’ director of mental conditioning, nearly every day. The two discuss his performances. Doolittle strives to glean the positive and extract lessons even when things go sideways. Breathing and counting help him stay centered and calm in tense situations. It all slows down.

“It buys me time to think about what I want to do,” he said. “There are nights where I come back in and I’m like: ‘I actually blew it there. I let it speed up on me, and I couldn’t slow it down.’ . . . There are nights where you see me out there messing with my glasses. I’ll untie my shoe. I’ll mess with something in the back of the mound. Just trying to reset my energy. Press reset.”

In some ways, Doolittle pressed that button when he arrived a year ago. He was a noteworthy piece when the Nationals added him as part of their bullpen overhaul, a former all-star with an electric fastball, but he came with questions. With some adjustments, those have been forgotten.