Manager Scott Servais termed it a "small tear" in his pregame media availability, but it's troublesome nonetheless. This is the second time this season the Mariners have placed last year's American League Rookie of the Year on the injured list.
Servais said Lewis had an MRI on Tuesday morning, and there was a thought that the injury was similar to the bone bruise in his knee late in spring training.
"It was different than what he had in spring training. Yesterday, the initial report that I got, it was similar to that. It was different. It was a meniscus issue. In spring training, it was not," Servais said.
Lewis appeared to get injured while trying to run down a fly ball from Sean Murphy in the eighth inning of the Mariners' victory Monday over Oakland. Lewis made a leaping attempt on the deep drive and left the game before the start of the ninth inning.
Lewis missed the first three weeks of the regular season before debuting April 20. After a slow start, Lewis had come on of late, hitting .303 with two doubles and two home runs over the past nine games.
"The sad thing was he was on a great routine," Servais said. "He was feeling really good. I know just talking to him daily it's the best his legs had felt underneath him. We certainly saw what he was doing at the plate and the bat was really heating up. So we're going to miss him."
Seattle recalled outfielder Taylor Trammell from Triple-A Tacoma to take Lewis' spot. Trammell hit .157 in 27 games with the Mariners earlier this season, but he was batting .384 with six home runs and 19 RBIs in 17 games since going down to Triple-A.
While Trammell was the primary center fielder when Lewis was out earlier in the season, the Mariners plan to have rookie Jarred Kelenic as the primary center fielder until Lewis returns.
"He's very comfortable in center field. He loves playing center. He's very capable," Servais said. "I thought he's made a couple of really nice catches in left field, plays he's made out there look easy that are not easy."
Seattle also activated left-hander Marco Gonzales from the injured list to start Tuesday night against the A's. Gonzales has been out since late April with a strained forearm. The Mariners optioned right-hander Robert Duggar to Tacoma.
The Associated Press contributed to this report.