Seattle called up left-handed reliever Ryan Rowland-Smith from Triple-A Tacoma to take Weaver's roster spot.
Weaver has been a flop after signing a one-year, $8,325,000 contract to be the Mariners' No. 5 starter. He is 0-6 with a 14.32 ERA in six starts. In 22 innings, he has given up 50 hits and 35 earned runs.
"Something just hadn't been right the whole time, and we really thought, in talking to him and in internal discussions, it was just a matter of repetition and getting out there and pitching," Seattle manager Mike Hargrove said.
"We finally pressed him on the issue."
The Mariners now believe it's a strength issue causing Weaver's problems. Weaver started on Thursday at Detroit, allowing six runs in five innings, including a pair of homers. He has not had a start this season in which he has allowed fewer runs than innings pitched.
Weaver, who spent 10 to 20 minutes before batting practice Friday speaking with his agent, Scott Boras, says the pop on his pitches is absent.
"It just feels like I'm putting 100 percent effort into every pitch to get minimal results," Weaver said.
It's the first time in his career Weaver has been placed on the disabled list.
"Jeff is a guy that won't complain about anything and just takes the baseball," general manager Bill Bavasi said. "It became obvious to us when he needed to reach back for that little something extra, his arm strength just wasn't there."
Once the inflammation in his shoulder subsides, Weaver will be placed on a strength program. Weaver doesn't know if he'll need more than 15 days, but he wants to be confident in his arm strength before returning.
"Is he a guy who is at full strength? And the answer is no," Boras said.
Seattle does have options if Weaver cannot regain the form that helped St. Louis win the World Series last year. Young ace Felix Hernandez is scheduled to return from a right forearm strain Tuesday, and Cha Seung Baek has pitched well in his four starts since being called up from Tacoma, including a complete-game win against Detroit on Wednesday.
Rowland-Smith was 2-2 with a 5.68 ERA in 11 relief appearances for Tacoma, and arrived in Seattle on Friday morning after leaving his teammates in Round Rock, Texas. Rowland-Smith has never appeared in a major league game.