That would be the rehab for Sizemore's right knee, which was operated on in October after he initially injured it in May. A year ago, the veteran was returning from microfracture surgery on his left knee and he eventually made his way back to the lineup in April after a 10-month recovery.
This past offseason, Cleveland declined its $9 million club option to retain Sizemore in 2012 before deciding to re-sign him at a lower cost. The Indians added him back to the mix with a one-year, $5 million contract that included another $4 million in performance bonuses.
Cleveland understood the gamble involved in keeping Sizemore in the fold.
"At the time we signed Grady, we knew that there was some risk," Antonetti said. "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."
One potential scenario -- the same one that came up when Sizemore was unavailable for Opening Day last season -- could have left fielder Michael Brantley shift to center. Manager Manny Acta stressed that the team plans on evaluating its other options for center throughout Spring Training.
Beyond Brantley, Acta said that candidates in camp who are possibilities for center field include Ezequiel Carrera, Aaron Cunningham, Ryan Spilborghs and Felix Pie. If Brantley is in center, those four players could be viewed as options for left field, along with Shelley Duncan and Fred Lewis. Shin-Soo Choo is the starter in right.
"We're going to have to see how those other guys play," Acta said. "That's what Spring Training is all about. We certainly are very comfortable with Michael in center. He is a center fielder, but we have more people in camp and we're going to see how things line up."
Sizemore, 29, injured his lower back while fielding a ground ball in the outfield, according to head athletic trainer Lonnie Soloff. At the time, Sizemore had been hitting and throwing with no restrictions, and his running program included a walk-jog routine and some light agility drills.
Under the circumstances, Sizemore was immediately shut down from baseball activities, and Soloff is not sure when he will be cleared to resume with such workouts. For now, Soloff noted that the only activity that Sizemore can take part in is stabilization exercises to strengthen the core and lower back.
The rest of Sizemore's knee rehab has been put on hold until his back heals.
"The challenge," Soloff said, "is that there's so many other compounding factors. The knee rehab had to be slowed down, if you would, so then any progress with that component has to be re-engaged after his back responds."
Soloff added that it was "tough to say" whether Sizemore would have been ready by Opening Day even without the added back injury.
Even so, the Indians believed that Sizemore was in a great position to have a solid comeback season after using this past offseason to rest and recover from his injury issues last year. Besides the knee injury, the outfielder also underwent an in-season procedure to correct a sports hernia.
Thanks in part to the health woes, Sizemore hit just .224 with 10 home runs and 32 RBIs in 71 games for Cleveland in 2011. Over the past three years, a variety of injuries have limited Sizemore to an average of 70 games per season. He played in at least 157 games between 2005-2008, earning three All-Star nods and two Gold Gloves in that stretch.
Antonetti made it clear that Sizemore's latest setback was not the result of pushing himself too hard in his comeback attempt.
"The rehab process is pretty structured," Antonetti explained. "There's a set progression through activities. Grady worked diligently to meet each of those benchmarks, but he was certainly operating within the rehab parameters. So it wasn't a question of him doing too much too soon or anything like that."
The Indians, who hope to contend for the American League Central title this season, can at least be encouraged by the team's fast start without Sizemore a year ago. The team ran out to a 30-15 start without having Sizemore in the lineup for the beginning of April and the end of May.
"He has been battling injuries the last couple of years," Acta said. "We've had guys like Michael step up. Now, you can see how important it was to add all those veteran guys to the Spring Training mix. Things can change quick.
"The main thing is, once he gets over the back issue here, he's going to continue to rehab his knee and at some point be ready. We're saying he's not going to be ready by Opening Day tentatively.
"That doesn't mean he's going to be out for a long period of time."
The Indians are hoping to avoid that kind of scenario.
"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."