ST. LOUIS -- Johan Oviedo’s Sunday was slated to be a simple one. The original plans were for him to oppose Kwang Hyun Kim in a simulated game at the alternate training site, where the right-hander would continue to sharpen some of the back-field experimentation he conducted during Spring Training.

Plans change.

The placement of Tyler O’Neill on the injured list incited a roster shuffle for the Cardinals, and Oviedo heard his name called to the big league roster. When Daniel Ponce de Leon slogged through the first inning, Oviedo was then called to begin warming in the bullpen.

By the time the Cardinals wrapped up their 9-3 loss to the Brewers at Busch Stadium -- dropping a rubber game to their National League Central rivals -- Oviedo had gone from expecting an afternoon flanked by only teammates and coaches in Sauget, Ill., to pitching 4 2/3 crisp innings in front of 13,176 fans.

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St. Louis’ No. 9 prospect (per MLB Pipeline) had a strong line in his 2021 debut: two hits allowed, two walks and four strikeouts, driving in more runs at the plate (one) than he allowed to Milwaukee on the mound (zero).

“I hope I can get more ABs,” laughed Oviedo, who became the first Cardinals pitcher with an RBI in his first career plate appearance since Mark Worrell in 2008.

The Cardinals will take RBIs from a relief pitcher, but they simply want more innings out of their 23-year-old right-hander, who was a dark horse to be in the rotation in Spring Training.

As manager Mike Shildt confirmed postgame the suspicions that grew earlier in the spring -- that St. Louis could feature a sixth man in the rotation in spurts between now and the All-Star break -- consider Oviedo’s stock to be on the rise.

“He gave us even more confidence than we already had,” Shildt said. “ … Ovi will be part of that plan.”

That confidence was kicked into high gear during Spring Training, when Oviedo was kept mainly to back-field games. The Cardinals already knew his fastball (which averaged 96.3 mph on Sunday) and his slider (which finished off three of his strikeouts) were plus pitches. They wanted him to hone in on a curveball and changeup, which came out in force against Milwaukee.

Last year, Oviedo pitched out of necessity, with the Cardinals needing innings after their COVID-19 outbreak. Even still, lessons could be learned.

“I would say it’s more about confidence,” Oviedo said. “Last year, I was more fastball and slider; I was not comfortable with my curveball and changeup last year.”

And last year, when Oviedo debuted at 22, he did so without fans at Busch Stadium.

Consider Sunday a debut redux.

“Just enjoy, look around and see how nice it looks outside,” Oviedo said, “and then just lock in and try to do what I always do, which is pitching.”

The Cardinals like what they got from Oviedo, but that such length was needed was not encouraging. Ponce de Leon allowed seven runs in 1 1/3 innings, most disappointingly walking Brewers starter Brett Anderson after intentionally walking the bases loaded in the opening frame. It was an ominous sign for a day that didn’t improve for St. Louis, with a baserunning error from Austin Dean derailing a potential sixth-inning rally.

"I felt like my arm was like 100 pounds,” said Ponce de Leon, adding that he never got the feel he needed warming up in the bullpen. “Not 100 pounds, but real heavy. I just could not get it out in front. Just using my whole body to get a strike, and I just couldn't get it."

With Sunday’s events -- all too familiar only nine games into the season -- the Cardinals have received more innings from their bullpen (an MLB-high 40 2/3) than they have from their rotation (38 1/3).

“It's not a sustainable model,” Shildt said. “The off-days have helped us, bullpen has been great, but we have to get deeper and further along with our starters so we can actually use our bullpen how we like to use it, instead of putting guys in positions just because they're most rested. That's not a great recipe.”