GOODYEAR, Ariz. -- The unfortunate cycle of injuries and setbacks that has plagued Indians center fielder Grady Sizemore, once one of baseball's brightest stars, churned out another blow of bad news on Thursday afternoon.

The Indians announced that Sizemore underwent a minimally invasive procedure on his lower back on Thursday morning, taking him out of the fold for the foreseeable future. According to the ballclub, the required recovery period for the center fielder is estimated between two to three months.

"I feel bad for him," Indians manager Manny Acta said. "Obviously, it seems like he can't catch a break over the last couple of years."

The surgery -- a micro discectomy operation -- was performed by Dr. Barth Green of the University of Miami School of Medicine. Sizemore is expected to return to Arizona from Miami in the coming days to continue the rehabilitation process.

The latest setback came during Sizemore's comeback attempt from a right knee injury, initially suffered while running the bases last May. Roughly three weeks ago, while fielding ground balls during routine outfield drills, Sizemore tweaked his lower back. When the pain worsened over the next few days, he ceased baseball activities.

The Indians originally classified the injury as a lower back strain on Friday, though the club never offered a projected timetable for return. Sizemore was forced to stop baseball workouts, as well as his rehab from the knee problem. Opening Day was quickly ruled out as a realistic target date for return.

Indians head athletic trainer Lonnie Soloff indicated on Thursday that the team knew a couple weeks ago that there was an issue with a disk in Sizemore's lower back. Cleveland opted to first attempt a conservative rehab program before seeking a second opinion about the possible need for surgery.

When Sizemore did not respond well to treatment, it was then determined that the micro discectomy procedure was the best route to take. Soloff said the operation involves removing disk material that was putting pressure on a nerve root. In Sizemore's case, the nerve problem created issues with sensation in a calf muscle.

Indians general manager Chris Antonetti said the team remains optimistic about the chances of seeing Sizemore return at some point this summer.

"It will be an extensive rehab process," Antonetti said. "But we still are hopeful we will get him back for the bulk of the season."

The back surgery represents the sixth operation in the past four years for the 29-year-old Sizemore, who was known for his all-out style of play in his prime years with the Tribe. Sizemore's decline since 2008 has included two sports hernia operations, surgery on his left elbow, microfracture surgery on his left knee, an arthroscopic procedure on his right knee and now back surgery.

While some of the injuries may be connected, Soloff said there is no way to know that for certain.

Soloff also noted that Sizemore will have a period of inactivity that will last for one or two weeks. After that, the center fielder will engage in an aggressive rehab process aimed at strengthening his lower back and knees. The recovery period announced by the team accounts for Sizemore's knee rehab process as well.

Sizemore has been limited to an average of 70 games over the past three seasons. Prior to that stretch, the center fielder played in at least 157 games on average from 2005-08. During that run, he made three All-Star teams and won two Gold Glove Awards and a Silver Slugger Award.

Without Sizemore in the fold, the Indians are left with a hole in their starting outfield rotation. Both Antonetti and manager Manny Acta noted on Thursday that the most likely scenario is having left fielder Michael Brantley shift to center. Shin-Soo Choo will open the season as Cleveland's starting right fielder.

The Tribe has a wide range of outfielders in camp to compete for left field and the fourth outfielder. The list of outfielders in camp includes Shelley Duncan, Aaron Cunningham, Matt LaPorta, Russ Canzler, Ryan Spilborghs, Fred Lewis, Felix Pie, Ezequiel Carrera, Thomas Neal, Chad Huffman, Trevor Crowe and Nick Weglarz.

Over the offseason, Cleveland acquired Cunningham and Canzler via trades and signed Spilborghs, Lewis and Pie to Minor League deals that included non-roster invites to attend Spring Training.

"There's going to be plenty of at-bats to have out there," Acta said. "You might not have seen it as a big deal during the offseason, but now it looks pretty big when you have a full bag of guys like that who have been in the big leagues the last few years and have experience."

That said, Antonetti did not rule out looking externally for more alternatives.

"Our scouts will continue to be out there evaluating other camps," Antonetti said. "We're always looking for ways to improve the roster, but we feel good about some of the guys that we already have in camp, and we're looking forward to seeing some of them compete.

"But, part of our responsibility is to make sure we understand who the alternatives are and if we have an opportunity to acquire them, and improve our roster, we'll certainly look to do that."

Last season, Sizemore returned to the Indians on April 17 following a 10-month recovery from the surgery on his left knee. He was slowed when the right knee injury flared on May 10. He also had a sports hernia procedure on July 21 before later going under the knife for the knee issue in October.

In between all the injuries, Sizemore hit just .224 with 10 home runs, 21 doubles and 32 RBIs in 71 games for Cleveland. When he first rejoined the Tribe in April, Sizemore went on an offensive tear, launching a home run in his first game back and posting a .282 average with 16 extra-base hits in his first 18 games.

It was a brief flash of the old Sizemore -- a yearly threat to hit 20 home runs, collect 90 RBIs and cross home plate 100 times. That early showing also aided the Indians' decision to retain the center fielder for the 2012 season, though Cleveland did not pick up his $9 million club option.

The Indians declined that option on Oct. 31, but agreed with Sizemore on a new incentives-based deal on Nov. 23. Cleveland re-signed Sizemore for a guaranteed $5 million on a one-year contract, though Antonetti declined comment when asked if the team also secured an insurance policy as part of the deal.

Sizemore's new contract also included the potential to earn another $4 million in bonus money based on plate appearances. Of course, it remains to be seen when Sizemore will again step into the batter's box.

For now, the Tribe will once again move forward without Sizemore in the picture.

And Sizemore will attempt yet another comeback.

"Grady's obviously frustrated," Antonetti said. "He wants more than anything to be out on the field contributing and helping the team win. This is a setback in his timetable for doing that, but I think he understands that, if he sees this process through and doesn't have setbacks along the way, he can still contribute for the majority of the season."