SAN DIEGO -- A month and a half into his big league career, Luis Perdomo was pitching like he belonged in Double-A, completely overmatched by the Major League hitters he was facing for the first time with San Diego.
To be fair, Perdomo probably did belong in Double-A. The 23-year-old right-hander had never pitched above Class A Advanced before he was selected in the Rule 5 Draft last December. But because of his Rule 5 status, the Padres had to keep him on their active roster for the entire season or surrender him back to the Cardinals.
So regardless of where he belonged, Perdomo was in the big leagues to stay. And he was getting hit hard, with an ERA hovering around 10. At the time, Perdomo's primary pitch was a four-seam fastball -- a pitch that touched 97 mph, but possessed hardly any movement.
"Luis has a very good four-seam fastball with good velocity and command," said pitching coach Darren Balsley. "But for some reason, the hitters were hitting it. He'd make some good pitches, and he'd still get squared up. ... He needed something else."
That's when a request came down from the front office.
Padres general manager A.J. Preller asked Balsley to begin working with Perdomo on a sinker. One afternoon in late May, Perdomo started playing catch alongside Balsley in right field. The pitching coach didn't change any mechanics; he didn't change the delivery. All he did was tinker with the grip on Perdomo's two-seam fastball.
"It wasn't anything scientific," Balsley said. "It was actually a pretty generic grip. The third one he threw, it was just like, 'There it is.'"
Said manager Andy Green: "I just remember hearing them come in, and the excitement they had about [Perdomo's sinker] and how good it was and how they believed it could be. I could sense it when those guys were talking, not even having seen it myself."
In an early-June start in Colorado, right-hander Andrew Cashner went down with neck stiffness and San Diego called on Perdomo. Coming into the game cold, he pitched 5 2/3 innings, allowing three earned runs. It wasn't a dominant effort by any stretch, but he pitched like he belonged.
"When I fell behind in the count and had to go to my four-seam, hitters were sitting on it," Perdomo said through a team interpreter. "They were ready. When I started throwing my sinker ... in those same situations, I was just getting ground balls."
More ground balls, in fact, than anyone else in the National League. Among NL hurlers, nobody with at least 100 innings has a higher ground-ball rate this year than Perdomo's 59-percent mark.
The effect of the pitch was clear as day. On June 5, Perdomo's ERA was at 10.38. Beginning with that relief appearance in Colorado, it dropped in 12 of his next 14 outings.
"There was never a lack of confidence," Balsley said. "But the day I saw the big smile was the day he learned the sinker and saw how much it sunk."
Added veteran reliever Carlos Villanueva: "He didn't get picked in the Rule 5 out of cuteness. There was something that they saw in him, that we all saw in him."
What, exactly, did the Padres see when Perdomo arrived? In Balsley's words: "Beautiful delivery, good arm. But I didn't think he'd be this -- and I'll use the word -- dominant, this fast."
No, the overall numbers aren't good -- Perdomo posted a 5.59 ERA through 34 appearances (19 starts). But he's shown flashes of true brilliance in the second half, leading San Diego to believe he'll play a pivotal role in its future. On Aug. 28 in Miami, he tossed the team's only complete game of the season, needing just 99 pitches, while racking up a franchise-record six double plays.
"Who would've thought, here's this kid, at the beginning of the year we couldn't get him through a full inning," Villanueva said. "And now he's probably our best starter."
Who would've thought? Well, maybe Villanueva himself. Early in the season when Perdomo was struggling, Villanueva made a point to tell Green about Hector Rondon. In 2013, Rondon was a Rule 5 Draft pick with the Cubs, who struggled mightily in the first half before a very strong finish.
Like Perdomo, Rondon had some early-season kinks to work out, before his true talent came through. There will be no early-season kinks for Perdomo to work out next year.
"I'm going to feel good knowing that I have a year under my belt," Perdomo said. "I've learned a ton this year. Now, going into next year, I know what it is like to have a full season. I know the hitters and I know the team and what I need to do."
All along, the plan was for the Padres to sneak Perdomo through a season in the big leagues before returning him back to the Minors, perhaps with Double-A San Antonio, where he'd continue to develop in relative anonymity.
That plan has long been scrapped. Perdomo has proven his Major League quality this season, and he figures to be a rotation mainstay for years to come.
"Being a Rule 5 pick who had never even experienced Double-A baseball, yes, this is an extremely quick [progression]," Balsley said. "But he's a coach's dream. He does everything you ask him to do. And he's going to be as good as he wants to be."
AJ Cassavell covers the Padres for MLB.com. Follow him on Twitter @ajcassavell. This story was not subject to the approval of Major League Baseball or its clubs.