SEATTLE -- The lack of fastball command that caused Brad Keller to throw 79 pitches into the fifth inning against the Mariners wasn’t as worrisome as the final pitch the Royals starter threw Thursday night.

After a sinker down the middle led to Kyle Seager’s home run in the bottom of the fifth inning of the Royals’ eventual 6-4 win over the Mariners at T-Mobile Park, Keller grabbed his right arm and exited shortly after with what was later diagnosed as posterior right shoulder discomfort.

The right-hander was placed on the 10-day injured list on Friday with a right lat strain, and while the Royals haven’t determined a timeline for Keller’s return, he’ll likely be down for a significant amount of time.

“There are still tests to be run, but right now, I’m not excited about this being a post-shoulder issue,” manager Mike Matheny said.

The Royals don’t yet have a grade for Keller’s strain; they’re waiting to get back to Kansas City before beginning further imaging in order to be consistent throughout the process, which will require multiple MRIs.

Matheny went back and watched Keller’s first inning from Thursday, when he needed 36 pitches and walked in a run. Nothing stuck out injury-wise, and there had been no indication that Keller was feeling pain in his previous starts.

The Royals’ Opening Day starter recovered well in the second and third, retiring all six batters, and felt more in sync with his delivery than what he had felt in the first inning.

“The things we’ve been working on since the All-Star break, trying to slow down, stay gathered over the rubber and then go down and throw it through the target, I just felt like I was blowing through all that,” Keller said. “All my cues that I usually hit were hitting early on, I blew through all of it. So really just a recipe for disaster. Just had to sit back and make the proper adjustments to get out of the inning.”

Keller started to feel shoulder pain in the fourth inning. He loaded the bases with two singles and a walk, but was able to escape the jam and went inside to get treatment before going back out for the fifth.

He got a quick out on Mitch Haniger’s fly ball to center field. But then Keller yanked a fastball in the dirt to Seager, and the pain soared through his shoulder. He tried to get the next pitch back in the zone, and Seager crushed it.

Keller massaged his arm after watching the ball fly out, and that’s when athletic trainer Nick Kenny came out with Matheny to get Keller out of the game.

“Fortunately, Nick saw something in that fifth [and realized] that 'there’s something going on here, let’s just go talk to him,'” Matheny said. “And the answer wasn’t the right answer when we talked to him. Fortunately, we got him out when we did.”

“They said that was enough,” Keller said. “It sucks, especially because I felt like after that first inning I really wanted to get into the sixth, felt like I had the pitch count to do it at that point. Going out there in the fifth with 75 pitches. It really sucked.”

Keller didn’t know the severity of the injury when he spoke with reporters Thursday night, but any time a pitcher exits with injury, it’s concerning. Especially with what Keller means to the Royals’ rotation this year and in the future, even with his struggles this season.

“I’ve never been in this situation before,” Keller said. “I don’t really know how it all works. But obviously I want to get back out there as quick as I can. That’s my goal. I don’t know the severity of it. I just know that, sitting here right now, it’s pretty sore, pretty tight."