OAKLAND -- When Kyle Muller experienced the euphoria of being named the A’s Opening Day starter during the final week of Spring Training, the rookie left-hander was on cloud nine. Bestowed with such a high honor, the thought of getting demoted at any point in 2023 never crossed his mind.

Opening Day could not have gone much better. Opposing Shohei Ohtani at the Coliseum on March 30, Muller went toe to toe with the Angels' two-way superstar and helped the A’s to a win with five innings of one-run ball. But baseball sometimes presents unexpected developments, and Muller soon found that out a couple of months later.

Following that successful first outing, Muller struggled with each turn that came in the rotation. After a rough start on May 22 in Seattle that ballooned his ERA to 8.04, a mark which ranked as the second-highest in franchise history for an A’s pitcher through his first 10 starts of a season, Muller was optioned to Triple-A Las Vegas.

“It’s humbling, for sure,” Muller said. “You feel like you’re on top of the world and then have some bad starts. But it’s part of the process and part of my process in getting better. It was good, because I got to go and figure some things out.”

Recalled on Saturday to start in place of a recently injured James Kaprielian, Muller showcased some of those improvements in Oakland’s 7-6 walk-off victory in 10 innings over the White Sox at the Coliseum. Working through early trouble, he limited Chicago’s offense to three runs on six hits and four walks with four strikeouts over five innings.

A theme of Muller’s first stint with the A’s was the difficulty of facing hitters a third time through the lineup. Saturday’s outing was a reversal of that issue.

After allowing three runs through three innings, Muller retired seven of his final eight batters faced, with the White Sox hitters that did face him a third time going 0-for-5 with a pair of strikeouts.

“Finishing the fourth and fifth innings with zeros, that has kind of been a thing for me,” Muller said. “Later in the game, the third time through is typically when I started to struggle. Being able to lock it in and finish strong was a really big positive.”

Another noticeable difference was Muller’s increased fastball velocity, which he said was the result of mechanical adjustments as well as long toss and weighted ball workouts in the Minors. Through his first nine starts of the season, Muller averaged 92.7 mph with his fastball. Against Chicago, that heater averaged 94.3 mph and maxed out at 97.2 mph.

Armed with extra ticks on his fastball, Muller threw it more than he ever had in a single start over his big league career: 53 of his 86 pitches. The uptick in velocity helped his secondary pitches appear more deceptive, particularly the curveball, which generated 14 swinging strikes and six whiffs on its 18 offerings.

“My fastball and slider were starting to blend and there wasn’t a big speed difference my last couple of starts in the big leagues,” Muller said. “Getting my heater back to where I could attack guys was huge. Not having to rely on my slider as much and picking and choosing advantageous spots where I could throw it makes the pitch better as well.”

It’s unclear how the A’s will move forward with Muller. While Kaprielian is likely out through the All-Star break, an upcoming off-day on Monday affords them a chance to adjust the rotation in different ways. Whatever they decide, Muller certainly provided an encouraging showing, looking a bit more like the pitcher the A’s were expecting to get when they acquired him from the Braves this offseason as part of the Sean Murphy deal.

“Overall, I thought he did a great job,” manager Mark Kotsay said of Muller. “We asked him to go down and work on some things. He did a better job attacking the zone. The breaking ball was better. The curveball had some finish to it. We’re still working on that changeup. But to go five innings and keep us in the game, that’s what we were looking for today.”