SEATTLE -- The A’s believe they have a future frontline starter in Kyle Muller. That potential is why they entrusted him as their 2023 Opening Day starter. His first full season in the big leagues, however, is on an alarming downward trajectory.

Through 10 starts, Muller has struggled to match the hype that surrounded him upon arrival from the Braves in the Sean Murphy deal this offseason as Atlanta’s No. 1 prospect. Monday’s 11-2 loss to the Mariners at T-Mobile Park was yet another difficult outing for the left-hander.

Muller dug the A’s into an early hole with five runs allowed on a pair of home runs over the game’s first two innings. He completed five innings. Jarred Kelenic's RBI single in the fifth put Seattle ahead 6-0 and marked the fourth time Muller has allowed at least six earned runs in a start this season.

“He left a lot of balls in the middle of the plate,” manager Mark Kotsay said. “When you throw over the middle of the plate, it’s tough to get away with mistakes. When you’re not able to execute on the corners or make the pitches that you need to make, good lineups take advantage of that.”

Allowing six earned runs over five innings, Muller’s ERA ballooned to 8.04, a mark which ranks second-highest in franchise history for an A’s pitcher through his first 10 starts of a season. Only Willie Adams (9.00 in 1997) had a higher ERA through his first 10 starts.

“It’s obviously not what I had hoped for,” Muller said. “It’s not what I know I’m capable of. I’ve learned a lot. I need to just keep getting better. Keep the nose on the grindstone.”

Part of an A’s pitching staff that leads MLB in walks, Muller has actually been an outlier in that regard. He allowed only two walks and stands at just 23 free passes in 47 innings. But his three strikeouts against Seattle and 33 on the year signal one of the main issues leading to his struggles, as he’s often unable to finish hitters facing two-strike counts.

Kotsay noted the absence of Muller’s changeup as another factor. It’s a pitch Muller had developed in recent seasons with Atlanta that appeared to be evolving into an offering that can come in handy when he loses feel for his breaking stuff. Of his 97 pitches on Monday, he threw five changeups and four went for balls.

“It’s definitely something we’re trying to mix in more,” Muller said of his changeup. “Last couple of outings, I haven’t been able to have a consistent release on it.”

With 2022 All-Star Paul Blackburn set to make possibly his final rehab start with Triple-A Las Vegas on Wednesday before returning for his season debut, the A’s will likely have to make a subtraction from their rotation within the next week. Based on Muller’s rough stretch that is going nearly on two months, he might be the odd man out. At this point, all he can do is hope for another opportunity to right himself.

“It’s been a rough go for Kyle,” Kotsay said. “We’ve seen him have a game or two that we felt like he was making improvements and going in the right direction. He’s decreased the amount of walks, but very limited strikeouts. The inability to mix the changeup in right now is really hurting him. [Against] right-handed hitters, everything is kind of coming into them and there’s nothing fading down and away. We’re going to continue to work on that with Kyle and try to make progress.”