Trevor Rosenthal signed with the Nationals in October after missing a full season recovering from Tommy John surgery. (John McDonnell/The Washington Post)

WEST PALM BEACH, Fla. — Trevor Rosenthal allowed himself to look, if only quickly, because he had earned it after spending 559 days away from a major league mound.

He had thrown back-to-back balls to Evan Mendoza of the St. Louis Cardinals on Tuesday afternoon. Then he reached back for a fastball that, once thrown, made him glance up at the video board beyond the right field fence. And there were those three numbers, in the perfect order, sneaking up on Rosenthal like a passing summer storm.

100 mph.

“I was kind of surprised at how hard I was throwing. I didn’t really want that, either,” Rosenthal said, laughing, a short while after he logged a scoreless inning in his first Grapefruit League outing of the spring. “I just wanted to kind of ease into it. But I guess it happens.”

It doesn’t happen for everyone. It actually happens for a very select few. But Rosenthal can throw a baseball really, really hard, and that is why the Nationals bet on him with a one-year, $7 million contract — flush with performance incentives — after he missed all of 2018 while recovering from Tommy John elbow surgery. Before the procedure and before he was released by the team he faced Tuesday, Rosenthal was an all-star closer who notched back-to-back 40-save seasons. After the procedure, the 28-year-old slipped away from baseball, spent 14 months in full recovery mode and resurfaced at a personal showcase in Los Angeles this past October.

One scout at the showcase recalled multiple evaluators calling their general managers just a few pitches into Rosenthal’s bullpen session. About 40 scouts attended in all. One was the Nationals’ Jay Robertson, who soon reached General Manager Mike Rizzo with excitement in his voice, triggering Rizzo into action. Rosenthal agreed to a deal Oct. 31, marking baseball’s first free agent signing of the offseason, and the Nationals had to wait more than three months to see him face a live hitter.

The results against his teammates, across the first few days of camp, were worth it. The results against the Cardinals were better.

“There was a big number,” said Nationals Manager Dave Martinez, who also peeked at the scoreboard once Rosenthal started pumping in fastballs. “What’s good about that whole deal is that he thought he was throwing 94-95 mph. Which is kind of where we want him to be. For him it’s just about utilizing his legs and having good mechanics.”

If Rosenthal and Martinez insist that he stumbled into a triple-digit fastball, that can only speak to the raw power of his right arm. His average fastball velocity was 98.6 when he was at his best, according to FanGraphs, in a 2015 season that ended with a career-high 48 saves, a career-low 2.10 ERA and a 17th-place finish in National League MVP voting. He threw his fastball 74.4 percent of the time that year, a regular rate across his six seasons, and mixed in a change-up and slider that both register in the high 80s.

Rizzo admitted to lingering concern after signing Rosenthal — noting Tommy John surgery’s 85 percent success rate — but felt good after he passed a scout’s eye test and necessary medical exams. Rizzo was sitting in the first row by the Nationals’ dugout Tuesday, foam cup in hand, as Rosenthal’s fastball reached 98 mph, then 99, then, finally, 100. The 10-pitch inning was over almost as soon as it started. It gave the Nationals a glimpse of what they could get in 2018, a hard-throwing, high-leverage right-handed reliever to pair with left-handed closer Sean Doolittle at the end of games.

If Rosenthal reaches his pre-Tommy John form or something close to it, he can fill that role and even spell Doolittle when Doolittle needs a rest. If he doesn’t, another surefire eighth-inning option has not presented itself.

“He’s going to be really important. He really is,” Martinez said. “Obviously he came out today and hit that triple digit, and it’s early. He feels good, and I want him to continue to feel good going into the season.”

Martinez still wants Rosenthal to wade toward 100 percent, thinking that’s the safest approach for the coming weeks. He will have to test himself on back-to-back days. He will have to feel out his recovery. He will have to, more than anything, go slow. That’s tough for the competitor in Rosenthal, who loves to throw hard and has been itching to since Aug. 16, 2017. That was his last major league appearance, just eight pitches against the Boston Red Sox at Fenway Park, and a lot has changed since then. He now has long blond hair flowing onto his neck. He is 18 months older. He is smarter with his body. He has to be.

But both he and the Nationals hope some of the old Rosenthal remains. They got a small taste of it Tuesday, and that won’t be nearly satisfying enough.

“[I need] to make sure I’m recovering and not going out and pushing through something that I could take the time now to take care of,” Rosenthal said. “That’s kind of been a problem for me in the past. I wanted to just keep going and going. But luckily with the staff we have here, they’ve been really conscious of stressing the importance. They want to keep me healthy. They want to do everything they can to help me out."

Rosenthal then reached up to adjust his hat, revealing the long scar running up the inside of his elbow. Tommy John surgery cuts careers in two, before and after, and there can’t be a stark difference between the pitcher Rosenthal was and the pitcher he is now. Not for the Nationals and their bullpen. Not for the unwavering expectations Rosenthal keeps placing on himself.

“But I’m part of the equation,” he continued. “I have to tell them how I’m feeling. So that’s important, too.”

Read more on the Nationals:

Nationals reliever Koda Glover is shut down with forearm strain

Patrick Corbin’s first spring start came far from West Palm Beach. That’s significant.

The Phillies need Bryce Harper much more than the Dodgers do

Dave Martinez, in second spring training, shows growth in shifted approach

Erick Fedde added 20 pounds hoping to be more durable — and the Nationals’ fifth starter