PHOENIX -- Without his Rule 5 status, Luis Perdomo would've opened the season in the Minor Leagues -- probably with Double-A San Antonio. It's unlikely he'd have been on the hill for a performance like Monday in which he used his sinker to carve up big league hitters in an 8-4 victory over Arizona.

But forget the hypotheticals. The reality with the Padres and Perdomo was always that he'd get a trial by fire in the big leagues.

And right now, Perdomo -- a 23-year-old rookie, who had never pitched above Class A until this season -- is making the absolute most of his opportunity.

Perdomo recorded his best outing of the season Monday night, allowing three runs -- two earned -- over six-plus innings at Chase Field. He pitched with supreme confidence on the hill, recording his first career start without a walk.

"He knows he belongs," said Padres manager Andy Green. "I no longer think there's a doubt. ... I thought maybe at the very beginning of the year, he felt that way. I think he knows that he belongs here, and the future's very bright for him. That's exciting to see."

Put simply, the development of Perdomo's sinker has turned him into a big league-caliber pitcher in the span of about a month. Over the past 30 days, Perdomo's 62-percent ground-ball rate is second in the Majors, trailing only St. Louis' Carlos Martinez.

That was never more evident than Monday night's contest, in which Perdomo induced 12 ground ball outs and just two in the air.

"Every hitter in the big leagues hits a straight fastball," said Perdomo. "At least with my sinker, I go out there and just try to get ground balls. For me, it's an important pitch to get outs."

When Perdomo was a member of the Cardinals' organization, he threw a sinker, but was told to stop because the pitch was too erratic.

Initially, the Padres figured Perdomo could get by with his four-seam fastball. But he got roughed up for much of April and May before the club started focusing on his downward movement.

"It's 96 [mph], but people square it up," Green said of the four-seamer. "That's what you were seeing earlier in the year. He was throwing four-seamers. It looks like it should go by people -- and then average hitters hit it.

"There was a great emphasis maybe a month ago to really make it move. ... They won't hit it in the air. It does not get lifted."

Green's only qualm with Perdomo's outing on Monday was that he didn't throw the sinker enough. Both RBIs -- a Paul Goldschmidt double in the first and a Jake Lamb triple in the fifth -- came when Perdomo tried to sneak a slider past them.

"It's just something that makes him very, very unique," Green said. "He still doesn't quite believe, but he's believing more and more."

On multiple occasions, Green has compared Perdomo to former NL Cy Young Award winner Brandon Webb -- high praise, sure, but a comparison Green is apt to make, given that he played behind Webb in the D-backs' system more than a decade ago.

But Green thinks Perdomo's sinker is different from Webb's, which tailed throughout its entire trajectory to home plate. In Perdomo's case, the ball doesn't really begin to dip until very late, making it practically undetectable until the last possible moment.

Said Green: "If we can get him to throw the sinker 95 percent of the time, he's not going to get hit."

That's something of a strange request for Perdomo, because the pitch is still relatively new. But he's learning to lean on it, nonetheless.

"I threw it before, and then stopped throwing it for a little bit," Perdomo said. "It's just now that I'm throwing it again, I'm throwing it with a lot more confidence. All the fastballs that I throw are sinkers -- none of them are four-seams. It's just more confidence at this point."

AJ Cassavell is a reporter for MLB.com. Follow him on Twitter @ajcassavell.