He is listed as day-to-day and it is not yet known how the injury will affect his status for Tuesday's All-Star game in Boston. Canseco, batting .276 with 69 runs batted in, was elected to start as the designated hitter.
Canseco, 35, came out Friday in the eighth inning against Florida because of a stiff back. He underwent an examination Saturday afternoon and, after arriving late to the ballpark, was given permission to go to his Miami home.
Canseco missed 12 games in June and July last year because of similar back spasms, but didn't miss a start after the All-Star break. He had two separate stints on the disabled list because of back ailments in 1997.
In 1996, Canseco underwent back surgery to remove fragments and also suffered a ruptured disc.
Houston Manager Larry Dierker is ready to return to the Astros after the All-Star break, one month after he had a seizure in the dugout and underwent brain surgery.
"I talked to him this morning," interim manager Matt Galante said. "He's doing good. He's still planning on being back right after the All-Star break. His first game will be next Thursday against Detroit."
Dierker suffered a seizure in the dugout during a game June 13, and doctors removed tangled blood vessels in his brain.
Jeff Montgomery, Kansas City's career saves leader, is going on the disabled list because of a hip problem and has no idea how long he'll be out.
Montgomery, 37, has 297 career saves, three short of becoming the 10th player to reach 300.
"Right now, it's the farthest thing from my mind. I'm assuming it's never going to be," Montgomery said. "Coming into the season, hopefully that was going to be a nice little celebration in April or May, something to remember down the road."
Reflecting the collapse of the entire Royal bullpen, he is 1-4 with a 7.54 earned-run average, converting only half his 10 save opportunities. As a group, Kansas City relievers have 13 saves and 21 blown saves.
Boston's Nomar Garciaparra sat out his eighth consecutive game because of a groin injury, but said he'll be ready to play Tuesday in the All-Star game at Fenway Park.
"I'll probably be in there starting, probably play an inning or two," Garciaparra said.
He aggravated his left groin on July 2 against the Chicago White Sox and hasn't played since.
Boston Manager Jimy Williams said Garciaparra won't play today at Atlanta in the final game before the break. But Williams said the club was "leaving the door open" for Garciaparra to play Tuesday.
Indian catcher Sandy Alomar, on the disabled list since May 11, left his rehabilitation appearance in Altoona, Pa., after one at-bat because of discomfort in his bothersome left knee. Playing for double-A Akron, he started at catcher, but left early. The game drew a record 7,001 fans to the Blair County Ballpark. . . . While the Mets and Yankees slugged it out on the field at Shea Stadium, some fans took the fight to the stands. Police issued 93 summonses for a variety of violations, from disorderly conduct to drinking in public, a spokesman said. But no arrests were made.