SEATTLE -- Manager Mark Kotsay views this stretch of playing each of the A’s four division rivals over the next two weeks as an opportunity for his young club to measure how it stacks up in the American League West for next year and beyond.

One game into that span, Monday’s 7-0 loss at T-Mobile Park to a first-place Mariners squad -- arguably the hottest team in baseball -- made it clear that there is a lot of work to do for the A’s.

Perhaps the biggest difference lies in each club’s starting pitching. The 740 2/3 innings by Seattle’s starters leads the Majors, while their combined 3.75 ERA ranks second only to Texas. Meanwhile, an A’s staff that has seen an Oakland-record 21 starting pitchers utilized holds the highest ERA (6.09) in the Majors and fewest innings pitched (601 1/3) in the AL.

Kyle Muller struggled to pitch deep on Monday night. Rewarded with a start after a couple of solid appearances out of the bullpen, the left-hander surrendered a leadoff home run to J.P. Crawford to set the tone for an outing in which he matched a season high with six runs allowed, on nine hits and three walks with six strikeouts in four innings.

It was the fifth time in 14 games (12 starts) this season that Muller has allowed six runs. One of those outings also came against the Mariners at T-Mobile Park, on May 22, turning in a similar line of six runs allowed on eight hits and two walks over five innings before getting optioned to Triple-A Las Vegas the next day.

“I felt like I was in the zone pretty consistently with all my pitches,” Muller said. “But I left some pitches over the plate to a good-hitting team, and they took advantage of it.”

With his ERA now at 7.67, Muller has not met the expectations from when he arrived in the Sean Murphy deal rated as Atlanta’s No. 1 prospect this offseason and was named Oakland’s Opening Day starter out of Spring Training.

Looking to improve his repertoire in the Minors with a new grip on his changeup, Muller unleashed it on Monday more than he had in his previous two outings, with some encouraging results. According to Statcast, he threw 16 changeups and generated five whiffs (swing and misses) with it. His downfall was too many fastballs and breaking pitches leaking over the middle of the plate, which was the case on the homers he allowed to Crawford and Julio Rodríguez.

“I threw more changeups today than I had in the past, and it’s definitely getting better,” Muller said. “They had a lot of righties in the lineup, so I think it was a good lineup to try it against. That’s a really hot-hitting team, and you figure out how good your stuff is when you’re throwing it to those guys. I definitely saw some positive things from it.”

The A’s offense did not give Muller much of a fighting chance, as it was limited to four hits and shut out for the 14th time in 2023, which is tied with the Mets, Tigers and Royals for most shutouts in the Majors.

Beyond it being the 94th defeat of the season for Oakland, what Monday and the rest of this three-game series will provide this rebuilding A’s team -- now featuring some of its promising young prospects -- is a chance to compete in a playoff-like atmosphere in front of near-sellout crowds.

“It’s a playoff environment, and a lot of these guys went through it for the first time tonight,” Kotsay said. “This is the start of it against these teams in the West fighting for the division. Tonight was an electric crowd. Our young guys got to experience it and see what playoff baseball is about.”

Muller added: “Myself included, situations and environments like this is going to be big. We all have a road map of where we want to be. Hopefully, it’s playing in front of big crowds like that at home at this time of the year and going for a playoff push. Getting a taste of that this year is pretty cool.”