PHOENIX -- Grady Sizemore's latest comeback has already stalled.

The Indians' oft-injured, hard-luck outfielder is hurt again.

Sizemore will likely miss Opening Day for Cleveland with a strained lower back, an injury the former All-Star suffered while fielding ground balls as he prepared for what he hoped would be a healthy season.

"I feel awful for the guy," Indians general manager Chris Antonetti said Friday.

The Indians revealed Sizemore's injury one day before they hold their first full-squad workout. The club re-signed Sizemore as a free agent in November after choosing not to exercise a $9 million option on him. But the Indians, perhaps out of some loyalty to Sizemore, decided to bring him back with a one-year, $5 million contract loaded with incentives that could have earned him an additional $4 million.

Now, there's no telling when Sizemore will be ready.

Antonetti, though, will never count him out.

"One thing about Grady is he is very tough minded," Antonetti said. "He is an exceptionally diligent guy. He will overcome it and I'm confident he'll be able to get back and still contribute."

Indians trainer Lonnie Soloff said Sizemore will not do any baseball activities as he's monitored over the next few weeks and undergoes treatment. Soloff said Sizemore had been doing well in his rehab following knee surgery and appeared on track to possibly begin the season on time.

Sizemore has undergone five surgeries the past two years and played in only 210 games the last three seasons because of injuries. His full-speed-ahead style may have caught up with the 29-year-old, who once played in 382 consecutive games and endeared himself to Cleveland fans with hustle that could turn a double into a triple or save an extra-base hit with a crash into the outfield wall to make a catch.

Sizemore played in only 71 games after beginning last season on the disabled list. He made three trips to the DL and underwent right knee surgery Oct. 3, just days after his season ended with a .224 batting average 10 homers, 32 RBIs and zero steals for the first time in his career.

Indians manager Manny Acta hasn't seen a completely healthy Sizemore since joining Cleveland two seasons ago.

"I know," Acta said. "(Former manager Eric Wedge) was a lucky man -- a blessed man -- to be able to see this very talented guy get out there every day and enjoy it. I haven't had that luxury so far, but I enjoy the human being and I've seen him from the other side. I've seen some of it over here, too. But, you know what? It's a long season. Just because he's not ready by Opening Day, doesn't mean he's not going to come on strong and contribute to our ballclub."

With Sizemore sidelined, the Indians will likely move Michael Brantey from left field to center. Shelley Duncan, Aaron Cunningham, Ryan Spilborghs and Felix Pie are among the team's options in left.

"We have Michael, who has been there the majority of the time the last two seasons for us and done a very good job," Acta said. "Also, we have some people in camp that might give us some different options."

The Indians, who were ravaged by injuries last season, have 12 other outfielders in camp. Cleveland opened last season 30-15 and played well in long stretches without Sizemore, so there's reason to believe they can survive again.

"We certainly want Grady on the field and we feel we're a better team when he's on the field playing and producing," Antonetti said. "But, obviously, at the time we signed Grady, we knew that there was some risk, even just around his knee rehab. So we tried to build in better quality alternatives maybe than we had at the start even of last year. Hopefully the guys that are here in camp can take advantage of the opportunity and earn a spot and, when the season starts, potentially contribute if Grady's not ready to go."