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It's not as cut-and-dry as it seems when it comes to the Pittsburgh Steelers and what they may have planned for the safety position this year.

When the Steelers drafted Shamarko Thomas in 2013, it seemed like he was destined to be the successor to veteran Troy Polamalu once Polamalu decided his football career was over. But Polamalu made that decision this offseason, and it's still not guaranteed that Thomas will be the one to replace him.

"Replace," too, is a difficult word, because there is no safety on the Steelers' current roster—or, arguably, in the NFL—who can do what Polamalu did in his prime. But the Steelers do need a new starting strong safety to join free safety Mike Mitchell in the secondary.

It seemed like Thomas was on the fast track to starting. As a rookie, he played 193 defensive snaps and had two starts, according to Pro Football Focus, and 29 combined tackles. But in 2014, Thomas was relegated to special teams duties and played only two snaps as a safety.

Injuries played a major role in both years, first with an ankle hindering him as a rookie and a succession of hamstring problems that cost him five games in 2014.

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Those hamstring injuries, in particular, are of note because they're injuries that can recur throughout a player's career. There are ways to treat and prevent them, and an offseason of training dedicated to improving hamstring health could do a lot for Thomas' starting chances.

But when a prevailing NFL cliche is "the greatest ability is availability," the Steelers may not trust that Thomas can be a 16-game starter this year.

One hint this may be the case is what the Steelers chose to do last year when Polamalu missed a number of games with a knee injury—turn to veteran Will Allen. Including the playoffs, Allen appeared in 13 games last year and had four starts, giving him a total of 320 snaps played—318 more than Thomas.

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Though an unrestricted free agent to begin the 2015 league year, the Steelers brought Allen back on a one-year, $1.05 million contract. This could be a sign that the Steelers are comfortable with the soon-to-be 33-year-old Allen in a stopgap starting role until they can figure out the position for the long term.

Just the same, it could be an indication that the Steelers prize Allen as depth only. Further, like Thomas, he is also an effective player on special teams.

Another option would be to move Mitchell to strong safety, though that would leave the Steelers trying to fill the free safety position. But it is something that Steelers head coach Mike Tomlin said in March he is "not opposed" to. Mitchell was the Steelers' second-leading tackler in 2014, with 71 combined, but he had just three passes defensed and zero interceptions and struggled as part of a new-to-him defense.

There's also the matter of the two torn groin muscles he played with in 2014 and has since underwent surgery to repair. The hope is that a healthy and more acclimated Mitchell will step up his game, whether at free or strong safety this year. Either way, he's a starter—it's just a matter of what his official position will be.

Someone to watch in this battle is rookie Gerod Holliman, taken by the Steelers in Round 7 of the 2015 draft. Holliman was the Jim Thorpe Award winner in 2014, given to the NCAA's top defensive back, thanks to his 14 interceptions. But tackling isn't a strong suit for Holliman, and it's a tough art to master.

Holliman will have all the help he needs to become a more well-rounded defensive back this summer during training camp, and he is a promising player. If he's a quick study, then he could prove to be a good candidate for a starting job.

Holliman's ball-hawking skills alone are quite impressive—especially with the Steelers defense totaling just 11 interceptions in 2014—and they make a strong argument for him seeing the field early in his career.

The Steelers' goal is likely to have Thomas ready to be a full-time starter this year and hope that he stays healthy. If he isn't ready—or if he falls injured again—they have in Allen someone they trust enough to play the position well. Meanwhile, Holliman can be coached up in his tackling technique and eventually find himself in a starting role, either at free or strong safety.

Before training camp begins in full swing, it's fair to pencil in Thomas as Polamalu's successor—but only lightly, and keep the eraser handy.