BALTIMORE -- The Orioles officially placed second baseman Brian Roberts on the 15-day disabled list Monday afternoon with a strained muscle in his abdomen. But the expectation is he will likely be out beyond that as the injury re-aggravated the herniated disk in his back, requiring a second epidural injection.

Roberts received the epidural on Monday and will have to rest for at least four to five days before resuming any baseball-related activities. He also underwent an MRI, with the test's results showing no further structural damage than originally thought. While the team initially thought Roberts' two injuries were not connected, manager Dave Trembley addressed the media again early Monday evening and said that further examination shows Roberts probably irritated his back with Friday's slide into second.

"He will be out the entire 15 days, and then [it's] a good possibility it may be longer than the 15 days," Trembley said. "Because he would have to start doing some baseball activities ... and in the information that was given to me, a rehab stint is possible, and probably likely."

The back issue is what originally sidelined Roberts this spring. He suffered a herniated disk in his lower back during offseason workouts in Arizona and only had 19 at-bats before the regular season started.

Still, Roberts and the team both pronounced him to be in playing shape, and the second baseman was starting in his fourth consecutive game on Friday when he injured his abdomen while sliding into the bag in the first inning on a successful attempt to steal second. Roberts walked to open Friday's game, and the impact of the bag hitting his midsection is what is believed to have caused the muscle strain. Roberts said he immediately felt the strain, and after scoring the game's first run, he did not return.

While Trembley originally stood firm in his stance that Roberts' back and abdomen injuries were not related, it now appears that the two have intersected. Monday's epidural was Roberts' second shot to help calm the inflammation in his back. He received his first epidural on March 15 when he flew to Baltimore to see back specialist Dr. Lee Riley.

A dynamic table-setter for the top of the Orioles lineup, Roberts hit .283 with a .356 on-base percentage and a .451 slugging percentage in 159 games last season. The 32-year-old hit 16 homers with 79 RBIs and a franchise-record 56 doubles. The latter number set the all-time single-season mark for two-baggers in a season by a switch-hitter.

Infielder Justin Turner was recalled from Triple-A Norfolk to take the open spot on the roster, but the team will likely use veterans Julio Lugo and Ty Wigginton to fill Roberts' role. The Orioles acquired Lugo during the last week of Spring Training in the event Roberts' back acted up, but Trembley said the job will not be Lugo's alone.

"It won't be a tryout, but there will be some different lineups based on matchups, based on what we feel that particular guy brings for that game," Trembley said.

"Obviously it is a major loss. We are going to have to pick up the pieces and fill in as best as we can. He is very valuable to the club and the guy that sets the tone offensively. I don't think you replace it; you just try to do the best you can to fill in when he's gone."

Roberts, who was batting .143 with one double at the time of his injury, is in the first year of a lucrative four-year contract extension he signed last spring. The Orioles committed to Roberts for $10 million a year through the 2013 season, one of the largest contracts on their books. Roberts, a two-time All-Star, is 17th on the stolen-base chart for active players.