Manager Lloyd McClendon also announced Friday that left-handed reliever Charlie Furbush, on the 15-day disabled list since July 8 with what was thought to be tendinitis in his left bicep, has a slight tear in his rotator cuff.
Furbush is scheduled to get a second opinion, McClendon said.
"When we get that second opinion, we'll formulate a plan," McClendon said. "It's highly unlikely that he will pitch again this year."
Beimel (2-1, 4.78 ERA) has allowed runs in five consecutive games. His move was retroactive to Thursday.
"It's hard for him to function," McClendon said. "He can't get extension, so he's spraying the ball and elevating up in the zone. I think this was a necessary move to try and get him right."
Rasmussen, acquired from Toronto on July 31 in the deal for Mark Lowe, was 2-1 with a 16.20 ERA in eight games with the Mariners before being sent down on Monday.
Left-hander James Paxton, sidelined since late May with a strained middle finger, is scheduled to begin a rehab assignment Sunday at Triple-A Tacoma. McClendon said it was hoped that Paxton would throw about 45 to 50 pitches in his first outing.