The 35-year-old, five-time All-Star paused after the awkward swing before his knees buckled because of pain. Sweeney tried to stretch his back at the plate before trainer Rick Griffin escorted him into the dugout and then the clubhouse.
The team reported Sweeney has spasms in his upper back. He will be re-evaluated Thursday.
Sweeney was starting for Russell Branyan. Branyan has had spasms in his lower back for six days but said he may return Thursday.
The Mariners said earlier on Wednesday they are going to be without starting pitcher Ryan Rowland-Smith for longer than originally thought.
Manager Don Wakamatsu said Wednesday that doctors have
discovered fluid in the elbow joint of Rowland-Smith's pitching
arm. The left-hander's rehabilitation will be delayed about a week
-- beyond April 26, the day he is eligible to return from the 15-day
Wakamatsu said the MRI result was good news because there was no
Information from The Associated Press was used in this report.