MILWAUKEE -- Ryan Helsley was supposed to spend Tuesday night on the mound at AutoZone Park, trying to keep the Brewers' top prospects in check when Triple-A Memphis took on San Antonio.

Instead, Helsley was at Miller Park in Milwaukee, getting his first taste of the big leagues after he was called up earlier in the day as part of a flurry of roster moves by the Cardinals who also summoned infielder Yairo Muñoz and right-hander Giovanny Gallegos from Memphis. Right-hander Mike Mayers and outfielder Tyler O'Neill went on the 10-day injured list, while utility man Drew Robinson was optioned to Triple-A.

Helsley learned of his first big league promotion late Monday night, after Memphis wrapped up a lengthy day of travel from Round Rock, Texas.

"We actually just got off the bus back in Memphis around midnight [Monday] night," Helsley said. "[Memphis manager] Ben Johnson asked if my folks were coming to watch me [Tuesday]. I told him they weren't because they had to work. He said, 'That's good, because you're actually going to pitch in the big leagues.'

"I didn't believe it at first, but it was really cool. All my teammates were congratulating me."

Helsley didn't have to wait long for his debut after Jack Flaherty's short outing in Tuesday night's 8-4 loss. Manager Mike Shildt called on the hard-throwing right-hander with two runners on and two outs in the fifth inning. Waiting for Helsley in the batter's box was the reigning National League MVP, Christian Yelich.

"I didn't try to think too much about it," Helsley said. "I just wanted to go right at him and attack from the start."

Yelich went on to homer, but Helsley recovered, getting out of the inning by striking out Ryan Braun, his first of four over 2 1/3 innings.

And, for good measure, Helsley singled in his first big league at-bat in the seventh.

"I think I was more nervous in the bullpen, but once I got out there, you kind of just go back to doing what you were doing all the way up to the point before you get there," Helsley said. "It was a lot of fun."

Helsley is the Cardinals' No. 9 prospect per MLB Pipeline. He potentially could have made it to the Majors last season, but he was sidelined with shoulder fatigue last June. He worked his way back from the issue to make a rehab start in the Gulf Coast League in early August, but he was shut down for the remainder of the season after that outing.

"I felt fine after the start in the GCL, but I threw a bullpen a couple days later and the bounce-back just didn't feel right," Helsley said. "There were only two weeks left in the season, so they decided to shut me down and take it into the offseason."

Helsley posted a 5.79 ERA in four Grapefruit League appearances this spring, and a 3.86 ERA in two starts for Memphis this season. Helsley will work out of the bullpen for the Cardinals, where manager Mike Shildt thinks he can be a valuable addition.

"He's a young man with plus velocity," Shildt said. "Out of the bullpen, he has every opportunity to hit 98-100 mph with some life to it. He demonstrated that in Spring Training. He's an asset we're looking forward to seeing."

Mayers and O'Neill were injured in the Cardinals' series opener at Milwaukee on Monday night. O'Neill sustained an ulnar nerve subluxation in his right elbow trying to make a throw home in the second inning, while Mayers strained his left lat while pitching in the sixth inning of the loss.

O'Neill called the injury a "freak accident," and while it was disappointing to learn he'd go on the injured list, he understood the team's decision.

"You don't want to mess with anything when it comes to nerves, you want to give it some time," O'Neill said. "I'm going to take a couple of extra days to rest it, get right and just go from there."

After slashing .254/.303/.500 with nine home runs and 23 RBIs in 61 games last season, O'Neill made the team out of Spring Training as a fourth outfielder and was batting .286 with a home run and four RBIs in 29 plate appearances.

"I just found my groove again over the last couple games at Busch [Stadium] and I felt good, too, down in Mexico," O'Neill said. "That's the nature of the game, though. Injuries happen."

Bader nearing return

Outfielder Harrison Bader was out of the Cardinals' lineup for the second straight game Tuesday after pulling his right hamstring on Sunday.

Bader took part in what Shildt described as a "light, light workout" before the game, and he is expected to get at least one more day before returning.

"He's still day to day, but we'll probably stay away from him [Tuesday]," Shildt said.