OAKLAND -- Carlos Gonzalez believes his injured right hamstring played a major role in Oakland's decision to keep him off the team's Opening Day roster in Japan and instead start him at Triple-A Sacramento.

It was then that the rookie outfielder realized a tight hamstring is serious business. So when Gonzalez left Friday's game in Chicago with tightness in the same hamstring that slowed him down during Spring Training, there was no guessing what the next step would be.

"I knew I had to rest it," the 22-year-old said before Monday's series opener against the visiting Mariners. "In Spring Training, I kept playing because I wanted to make the team, but it just got worse and I think that's why I missed the Japan trip."

Sitting out the weekend's games against the White Sox was rough, Gonzalez said, but not nearly as bad as it would have been to miss more.

"I don't want to miss more than a week," he said. "If you don't keep an eye on it, it's probably going to come back."

With that mind-set, Gonzalez made sure to make the most of ice wraps, running drills and everything in between to gain a spot in Monday's lineup, which made a late appearance in the clubhouse because of the outfielder's unknown status.

"We wanted to make sure he was ready to go," manager Bob Geren said. "He ran today and felt fine, so he's in there."

Also in the lineup was Ryan Sweeney, who was kept out of Sunday's game because of a swollen left ankle before entering the game as a pinch-hitter. The rookie outfielder's ankle has become somewhat of a nagging injury that began June 26 while sliding into home plate against the Phillies.

Sweeney and Gonzalez make up the A's outfield injury update list, but there are still plenty of other question marks roaming around Oakland's clubhouse. On Monday, Geren provided some answers about the ever-changing roster.

At third base, Jack Hannahan will continue filling in for Eric Chavez, who was placed on the 15-day disabled list Wednesday with right shoulder inflammation. Gregorio Petit, who was recalled from Sacramento on Saturday, could also get some playing time at the hot corner. Then again, he may be too busy a few steps away at the shortstop position.

Bobby Crosby, who had started each of the A's first 84 games at shortstop before sitting out with a strained left hamstring July 3, will be out until after the All-Star break. He was placed on the 15-day disabled list Saturday, retroactive to July 3, and is expected to return to the team's 25-man roster when eligible.

Petit was slated in the shortstop slot for Monday's game, but he could soon be vying for playing time with Donnie Murphy, who is day-to-day with a sore right elbow, which was the cause for his month-long DL stint beginning in May.

"Donnie's available, but he's not feeling great," Geren said. "He threw earlier and wasn't ready to start, but he said he could go in there if I need him."

Across the diamond, don't expect to find Mike Sweeney back at first base anytime soon. The A's veteran, who was placed on the 15-day DL on May 30 with left knee inflammation, had surgery on both of his knees June 11 and is not likely to return until August.

Meanwhile, Geren said veteran Frank Thomas could be back with the team doing baseball activity any day now. The Big Hurt, who has been out since May 29 with right quadriceps tendinitis, was scheduled to have an MRI on Monday to reassess the injury.

Reliever Joey Devine's rehab was stalled a week because of a setback with his right elbow, which has had him on the DL since May 30. The A's were hoping he'd be able to throw a simulated game Monday, but the right-hander will continue keeping relief pitcher Keith Foulke company on the DL.

Foulke was placed on the DL on Friday with right shoulder inflammation, setting in motion the team's decision to recall lefty Jerry Blevins from Triple-A. Foulke's DL trip -- his second of the season -- represents the 18th time the A's have used the DL this year. The number is four short of the Oakland record of 22 that was set in 1992 and matched last year.