Sean Doolittle is done for the season. But he wants a different ending in Washington.

Now he disappeared down the tunnel. The reality was that that moment — 9:29 p.m., on Sept. 10, 2020 — could have been his last in a Washington Nationals uniform. The weight of it crept up in the coming days, after an MRI confirmed he was out for the season. He could hardly move around his D.C. home, leaving him to binge-read a mystery novel and think.

A four-season run started with a trade Doolittle didn’t want. He’d been a member of the Oakland Athletics organization for a decade. He felt they were on the cusp of contending. Then he took to Washington in ways he didn’t expect. He and his wife, Eireann Dolan, were married here in 2017. They’ve connected to the community through a number of charity efforts. On the field, Doolittle was an all-star in 2018 and a World Series champion in 2019. He also battled injuries and some of the worst innings of his career. Through all of it, he grew.

And now, at the end of a season played through the novel coronavirus pandemic, there are two paths for his future: He could leave the Nationals without a proper send-off. Or Doolittle could re-sign with Washington in free agency and continue his rebound next spring.

He’s really hoping for the latter.

“That would be the best-case scenario,” Doolittle told The Post on Monday. “It’s a little bit nerve-racking, for sure. Just going through this process I don’t want to like get my hopes up, or have favorites or spots like oh man, ‘I really hope this works out with this team’ or whatever … what, what?”

A voice was chiming in from the background.

“Eireann is sitting across the table from me and says, ‘I want to stay here,’ ” Doolittle added with a laugh. “She says: ‘Print it.’ ”

Dolan says something else, the words a bit muffled, and Doolittle again played middle man in his own interview.

“On the record. Print it,” he repeated. “She’s serious.”

Doolittle turns 34 this month. He has never been a free agent or gone through arbitration. He signed a five-year, $10.5 million extension with the Athletics in 2014, designed for stability and a steady income. It included two club options — for $6 million in 2019, $6.5 million for 2020 — and the Nationals exercised both. The second made him an integral part of last October’s championship run.

But then came the pandemic, and Doolittle struggled. He couldn’t throw off a mound during the sport’s four-month shutdown. When it restarted in July, his body was fatigued and he developed bad habits in his delivery. He wasn’t pushing off the mound the right way. His fastball velocity sagged before settling into the high 80s, and the early results were troubling.

The lefty yielded seven hits and five earned runs in the first two weeks of the season. Three of the 18 hitters he faced took him deep. It was “really scary,” he says now, to feel fine physically and be all but serving batting practice to opponents. So the Nationals put him on the injured list, citing right knee fatigue, and gave him 13 days in Fredericksburg, Va., to reset.

“There were weeks there where I thought it was just gone forever,” Doolittle recalled. “I was like, ‘Uh, well I just throw 88 with no life now. I’m going to have to figure out how to get guys out again.’ I thought about learning how to throw a sinker, like, could I just dive bomb sinkers maybe and be a sinker-curveball guy? If these are my mechanics now for some reason, can I use them to create a new version of myself?”

It wouldn’t come to that. Doolittle found a fix, a tweak to his lower-half mechanics, that restored his fastball velocity and movement. The hour-long drives to Fredericksburg, straight south down Interstate 95, became something to anticipate. He had long phone conversations with Manager Dave Martinez. He tore through video. He was energized by catches with top pitching prospects, whose fastballs stung inside his mitt.

In the spring, while the pandemic raged, when the country erupted with protests after the death of George Floyd, who was killed while in police custody, Doolittle had wondered how long he would play. He has many interests beyond baseball. He was initially unsure about participating in 2020. He could have finished out his contract and hung it up on his terms.

But this season — however frustrating, however exacting on his mental health at times — reconnected him with baseball. He says that returning in August, pitching four scoreless innings, being in the bullpen and clubhouse, is some of the most fun he has had. He makes that clear when he explains why he wants to stay in Washington. He first mentions Martinez, then the strength and conditioning staff, then his teammates, then wanting to reward everyone who helped him through the summer.

“This just reminded me how much I love it, and how I want to continue with baseball as long as it will let me,” Doolittle said. “With the quarantine and the negotiations and covid, we weren’t sure if we were going to play at all, and then I land on the IL and I’m so ineffective that it’s scary. It scared me. Some doubt starts to creep in and then to be able to get it right, and come back and have the support of everybody behind you …

“I don’t know, it was absolutely a roller coaster of emotions. But I’m very grateful for it.”

Now comes the hard part — being judged on an uneven performance. Doolittle plans to live wherever he can best rehab his oblique this winter. He wants to send tape and velocity numbers to teams as soon as possible. In a recent news conference, Nationals General Manager Mike Rizzo wouldn’t discuss Doolittle’s pending free agency. But he did note the reliever’s stark improvements, adding that he believes there’s even more in Doolittle’s tank.

The first step for Doolittle was believing that himself. The next, in a perfect world, would be proving it in Washington.

“This has been our home for four years,” Doolittle said. “This city means so much to us.”