DETROIT -- Brusdar Graterol stared towards third base as he secured the grip on his two-seam fastball inside his bright red glove.

He turned his head towards his catcher, Jason Castro, and without pause, raised his left leg to begin his easy delivery and unleash one of the most anticipated pitches of the season for the Minnesota Twins.

It thudded into Castro’s mitt at 99.6 mph with armside movement. With that, the second-hardest pitch by a Twins player this season officially signaled the arrival of the organization’s top pitching prospect.

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“My mentality was, 'Hey, let's go enjoy it when you get in the game,'” Graterol said. “That's it."

Only days after his 21st birthday, Graterol, ranked baseball’s No. 54 overall prospect by MLB Pipeline, received his highly anticipated first Major League callup as part of Minnesota's first wave of roster expansion on Sunday. He immediately entered in the ninth inning of Sunday’s game with a five-run lead and pitched a scoreless frame to close out the Twins’ 8-3 victory over the Tigers at Comerica Park.

"When a guy is throwing 100 miles an hour, you know,” manager Rocco Baldelli said. “And we knew coming in, obviously, this being his first outing, everyone was kind of locked in and paying attention. It's exciting. It's a great moment. And he went out there and got the job done."

In another welcome sight for the Twins, Byron Buxton entered the game as a defensive replacement for the ninth inning in his first appearance since Aug. 1. Max Kepler had been removed from the game earlier as a precaution with tightness in his left leg.

Nobody really knew what to expect from Graterol -- not Baldelli, not pitching coaches Wes Johnson and Jeremy Hefner, and not even the 21-year-old’s new teammates, many of whom had only heard stories and rumors about the young fireballer and his triple-digit fastball.

Graterol made a quick impression, following the 99.6 with three more fastballs of 99.3, 99.0 and 98.5 mph to blow away Dawel Lugo, his first Major League hitter, with a four-pitch strikeout.

"Oh, yeah, that felt good,” Graterol said. “Great. I mean, great. For my first strikeout right here in the big leagues, wow. That was incredible."

He ran into a little bit of trouble on the basepaths when Brandon Dixon poked a slider to right field and Jordy Mercer legged out an infield single on a high chopper behind the mound, but he reared back again for 99.6 mph on his final pitch, which resulted in an inning-ending double play off the bat of Jake Rogers.

Graterol didn’t quite break the 100 mph barrier for the first time in his Major League career, as that 99.6 mph offering to begin his outing matched his highest velocity of the game. But he maintained that velocity -- his slowest fastball was clocked at 98.5 mph -- and most important, he threw strikes on 10 of his 14 pitches.

“The radar gun doesn't mean a ton to us in the dugout,” Baldelli said. “The results and the execution of the pitches, that's what matters. It's not easy to lock in in the ninth inning, pitching in your first game in the big leagues. He went out there and threw quality strikes over and over again.”

And really, it’s hard to be picky about a guy that’s still lighting up the radar gun and was executing his pitches well. Johnson was even impressed with Graterol’s poise when he made a mound visit in the ninth inning to check on the young fireballer’s nerves.

“[The] slider was OK -- it's got to get a little tighter,” pitching coach Wes Johnson said. “But overall, my gosh, come on, man, the guy was throwing 99 [mph]. He was good."

It doesn’t look like the nerves will be a problem. Graterol always understood that there might be pressure to live up to the lofty expectations, and he has embraced that pressure throughout his journey to the Major League.

"Oh, yeah," Graterol said. "Every time. All the time. Every moment."

Though Graterol has been a starter throughout his Minor League career and posted a 5-0 record with a 1.89 ERA for Double-A Pensacola to begin the year, the Twins moved him into a relief role following his recovery from a right shoulder impingement in August. They fast-tracked him to Triple-A Rochester to see if he could be a factor for the Twins in September and brought him up after four appearances for the Red Wings.

Since his transition, Graterol had allowed three runs -- all on one homer -- while collecting 11 strikeouts and four walks in 10 1/3 innings. His fastball is consistently capable of touching the triple digits, and Graterol feels that it has played up in shorter stints, culminating when the right-hander reportedly hit 103.8 mph during one of his relief outings in Pensacola in August.

"That was a surprise for me, too," Graterol said. "When I threw it and I saw the board, it was like, 'Oh my god, really?' So I'm happy and ready."

The fastest pitch recorded by a Twins player in the PITCHf/x era was a 101.0 mph fastball by Juan Morillo on April 27, 2009. Morillo is, in fact, the only Twins hurler to have crossed the triple-digit threshold since pitch tracking began. With Graterol in the mix, look for that to change quite soon.

“I'd only heard about him,” Jake Cave said. “So he got out there and the first pitch he threw was like 99 or something. It's pretty cool. You look at a lot of, like, the really, really good teams. They have guys in the back of their 'pen that throw really hard. Not always saying that they're the most effective, but now we have a guy that throws really, really hard that's in our 'pen, too. So I think that's pretty cool."