Trevor Rosenthal’s first exposure to enormous baseball pressure came against the Washington Nationals in a 2012 National League Division Series when he was a 22-year-old rookie. Fearless and overwhelming in three appearances, he faced 11 Nats, got 10 outs and fanned six, including Bryce Harper and Ryan Zimmerman twice each.

That began one of the best streaks of October pitching by any reliever ever. In eight playoff series for the St. Louis Cardinals over five seasons, his ERA was 0.69 with 42 strikeouts against 11 walks in 26 innings. In four World Series games, his ERA was 0.00. How many relievers, except Mariano Rivera, have handled pressure better and more often?

“Wow, wow, wow. . . . It’s easy to forget,” Nationals reliever Sean Doolittle muttered soon after Rosenthal, only 29, was given his unconditional release Sunday after a team-torturing stint in which he had almost exactly as many walks-plus-hit-batters (18) as he got outs (19) and had a 22.74 ERA in 12 games.

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Rosenthal’s nadir came Saturday when, called to protect an 8-4 lead in the seventh inning at Nationals Park, he walked all three Atlanta Braves he faced — his last pitch so wild it went above the hitter’s head. That image infected the relievers after him. All of Rosenthal’s runners scored, plus one more, with rookie Tanner Rainey on the hill. In the eighth, four more scored as Joe Ross blew a 9-8 lead and lost the game.

In a purge that felt similar to the cold-blooded NFL practice of “cutting the field goal kicker” after a brutal loss, the Nats arrived at the ballpark Sunday to find Rosenthal no longer a National and Ross sent to the minors. Whether that game, and its disturbing images, change the Nats’ season from hot to cold will be discovered in the next couple of weeks as the Nats play 12 straight games against some of the worst teams in the majors.

But a red-hot Nats team on an 18-7 run going into Saturday night has now had its momentum slammed in the face with another tough 4-3, 10-inning loss to Atlanta on Sunday, with Rainey giving up the two-run game-losing homer. Instead of a three-game series the Nats might have swept, the Braves are 8½ games ahead of the Nats in the NL East, not 6½ or even 4½ , which seemed more likely.

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The whole first half of this season has been played under the Rosenthal cloud of gloom. He was the setup-man domino that, once it fell, helped topple a whole bullpen to an unthinkable 6.29 ERA. Also, the Nats, both General Manager Mike Rizzo and ownership, which didn’t want to eat Rosenthal’s $7 million salary but ultimately will, may have balked at signing free agent Craig Kimbrel — who eventually signed with the Chicago Cubs for a stunningly low $43 million — because they thought Rosenthal, coming back from Tommy John surgery, would find himself.

The whole Rosenthal experience has left the clubhouse stunned and sad. The players aren’t happy to see him go, as fans certainly are. They’re still mystified.

“Trevor is an incredible athlete. And he had an awesome attitude,” Doolittle said. “We all loved him. We saw how hard he worked. He was asking people to watch film with him [to find flaws]. He went to the minors [when asked]. He never moped, which can put negative energy into the whole [bullpen] group.

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“He always wanted to pitch, didn’t care about a [status] role. So much so that by the fourth inning you’d see him up stretching when some guys might be hiding. He always sat beside [bullpen coach] Henry [Blanco] and the phone.”

To his teammates, Rosenthal will always be the former 48- and 45-save star who still had his 100-mph fastball, wicked slider and even a decent change-up to put in the back of hitters’ minds. But he completely lost his mechanics, or his poise and his confidence — in other words, he lost all control. The Nats felt for him so much because they all saw themselves in him.

“Coming back from a big surgery, you never know what the body will do,” said outfielder Adam Eaton, who, after massive knee and ankle injuries in 2017 still figures out new techniques to make defensive plays that used to be natural. “For me, ‘How do I walk? How do I do everything?’ For him, maybe, ‘How do I throw?’ ”

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Volunteering to pitch in any middle inning, any mop-up role, impressed teammates. “That’s what you have to be,” Eaton said. “But not many are.”

To Nats fans, on the absolute opposite hand, Rosenthal is likely the worst flop in the 15-year history of the team in D.C. as well as the player whose departure may cause the most cheers. Except perhaps for Jonathan Papelbon, who choked Harper in the dugout. Okay, check that — maybe not Papelbon anymore, either.

In just a dozen games, Rosenthal became a one-man cascade failure that not only decimated the games he was in, but games both before and after he appeared as the Nats constantly tried to fix him, forget him, farm him to the minors or fumigate the brains of the rest of a fragile bullpen.

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Rosenthal also constantly cast Manager Dave Martinez in his worst novice light as an inexperienced handler of pitching, especially bullpens. No moment hit Martinez’s reputation harder than Rosenthal’s last night as a National on Saturday.

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Martinez, hoping to mend the confidence of a potential star, called Rosenthal when baseball-reference.com says the Nats had a 95 percent chance to win.

Oh, if only the old Trevor, who materialized in bullpen sessions and who even got a key out Friday, would reappear, then the Nats in a wild-card spot, or even catching the Braves, would seem so sensible.

That’s how Martinez thinks — always with “positivity.” A manager needs to envision a winning plan. But the boss’s first job is to imagine everything that can go wrong — and its impact in all directions. That’s the real risk-reward equation. What would the cascade effect of Bad Trevor be? And was it worth the risk?

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That “walk, walk and walk” answered the question.

Rosenthal had the grace to speak after his debacle Saturday, but not to be candid, unless he is in denial that the game has gotten in his head.

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“After I came out of the inning it was really a small mechanical tweak,” he said. “It’ll be something [where I] come back tomorrow and I’ll be able to figure it out fairly easily.”

But there was no tomorrow.

The Nats’ failure in the Rosenthal experiment is now total, especially because his teammates assume it’s just a matter of time until he clicks elsewhere.

Perhaps the departure of Rosenthal turns a page for the Nats. But there may be chapters left for him.

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“Great person, great family,” Eaton said. “I hope he gets it together. He probably will.” Then Eaton thought. “And I won’t like to face him.”

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