"Guyer is going to end up needing surgery," said manager Joe Maddon. "It's his shoulder. I'm not exactly sure what's going on with that."
The outfielder will have surgery to repair a torn labrum in his left shoulder (non-throwing) on Thursday.
It's the worst-case scenario for the Rays, who were expecting Guyer to possibly return next month. Instead, he's out for the year.
"When it happened we just didn't know," Maddon said. "It was one of those things that when you talked to him about it there was never a strong feeling one way or the other, let's get more opinions, and then finally it's gotten to this point. I didn't know what to think when I first heard about it."
Maddon was not sure how long the expected recovery time would be.
"It's not brief," he said. "I don't know."
Guyer is one of four outfielders on the disabled list, along with Desmond Jennings, Sam Fuld and Brandon Allen. They've combined to miss 86 games this year.
Jennings, who missed his 13th game on Friday with a left knee sprain, is likely to begin a Minor League rehab assignment next week.
"It looks like he might be able to get something going by the middle of next week," Maddon said. "When we go back home on Monday, [he'll begin] more concerted baseball activity in a practice sense and then get out on a field and play in a game, probably by the middle of next week."
Guyer played in just three games this year. He homered in his last game, a solo shot in a 5-3 loss to Baltimore on May 12. Guyer played in 15 games for the Rays in 2011, his first season in the Majors.
The development could provide an opening for Hideki Matsui, who signed a Minor League contract on April 30. In nine games with Triple-A Durham entering Friday, Matsui was 4-for-33 with four RBIs. Still, it could take some time for Matsui to return to the Majors.
"He's doing well, he's not quite ready yet, but he's getting real close," Maddon said. "That's just the general consensus. He's feeling good, he's getting better, he's seeing the ball, but he's just not quite ready yet."