SEATTLE -- One day after saying that he was not at 100-percent health, Justus Sheffield was placed on the 10-day injured list with a left forearm strain on Wednesday, a concerning development given the possible severity of the injury and the prolonged struggles he’s had over the past month.

In a corresponding move, right-hander Erik Swanson was reinstated from a 34-game stint on the IL and returned to Seattle’s bullpen.

Sheffield described his issue on Tuesday as a general body injury, but he said that it wasn’t arm-related. Asked about it after Wednesday's 5-4 loss to the Yankees, Mariners manager Scott Servais didn’t have a thorough elaboration, saying that he was awaiting more detail from the club’s athletic training staff and doctors.

“How long it's going to take him to get back or get completely healthy, I'm not sure,” Servais said. “Again, that decision was made just before game time. … Hopefully, he won't be down too long. But there's something that's not right with Sheff, and we want to make sure he's 100 percent before he gets a chance to get back out there again.”

Sheffield gave up six earned runs over 1 2/3 innings, the shortest start of his big league career, in a 12-1 loss to the Yankees on Tuesday.

The Mariners must now determine what to do with Sheffield’s rotation spot on Sunday against the Angels and beyond.

Right-hander Robert Dugger seems to be an expected candidate, unless the Mariners add a Minor League starter to their 40-man roster, which seems unlikely for Sunday. Dugger would patch the problem for now, as he could fill in with a three- or four-inning start. But if Sheffield is out for an extended period, Seattle's starting-pitching depth -- which has been a concern all season -- could be even thinner.

The Mariners dropped from a six-man rotation to five after right-hander Justin Dunn went on the 10-day IL with a right shoulder strain on June 18. Dunn is nearing a rehab assignment, but he’s still at least two weeks away.

Santiago’s appeal to be heard Thursday
Left-hander Héctor Santiago’s appeal will be heard by MLB special adviser John McHale Jr. on Thursday in Seattle.

Santiago has been allowed to remain with the Mariners and continue playing since he received a 10-game suspension on June 29. Santiago became the first -- and so far, the only -- player to be ejected for using a grip-enhancing substance since MLB announced on June 15 that it would begin enacting stricter protocols and disciplines on "sticky stuff."

The Mariners and Santiago have maintained that the substance in question was a mixture of rosin, which is legal, and sweat.

“We'll keep our fingers crossed that something good comes out of that and see where it goes from there,” Servais said.

If Santiago’s suspension is upheld, the Mariners would not be allowed to replace his spot on the 26-man roster, which, given the developments on Sheffield, would put them in even more of a bind.

Santiago has emerged as a reliable multi-inning reliever out of Seattle’s bullpen and has pitched twice since being suspended. He was stretched out to 76 pitches in his most recent outing on Tuesday.

Servais maintains optimism on Lewis’ return
Outfielder Kyle Lewis has been rehabbing at T-Mobile Park for more than a week now, though he’s yet to ramp up any baseball activity. Servais has been optimistic on a 2021 return for the reigning American League Rookie of the Year, and he reiterated that stance on Wednesday.

“He's definitely going to be back this season,” Servais said. “I feel very confident in that. I know he's working his tail off. He's not picked up any baseball activity yet, and until they really put a bat in their hand in the cage or around the outfield a little bit, I really don't get into the timeline and things like that, so I don't have anything definite.

“I would suspect we would see him at some point. I'm hoping to get him back again at some point in August, I guess. … He is going to have to go play some games and get some timing down and things like that, so we're a ways away from there.”