SANTA CLARA, Calif. -- Before the San Francisco 49ers could even begin training camp practices, one of their most pivotal players for 2017 suffered a hamstring strain that has the team listing his availability as a week-to-week proposition.

Free safety Jimmie Ward suffered the injury during San Francisco's conditioning test on Thursday, and the team placed him on the physically unable to perform list soon after. That means Ward will count against the 90-man roster, but he can be activated at any point before the cut-down deadline.

Niners coach Kyle Shanahan didn't seem to think the injury would hold Ward back too much, but he emphasized the importance of not rushing the safety to return.

Jaquiski Tartt has appeared in 30 games, with 14 starts, for the 49ers, who like how he has progressed. Kyle Terada/USA TODAY Sports

"He did it in our conditioning test, which was unfortunate for him and for us, obviously," Shanahan said. "He tweaked it halfway through it. I don't -- we don't think it was a full pull or anything, but we want to be smart and safe with it. You never know with those things how long they'll take. Don't think it's anything too serious but want to definitely be safe with it."

Losing Ward now is a big blow, considering he's transitioning to free safety from cornerback in coordinator Robert Saleh's scheme. While Ward played free safety in college, the importance of the position in Saleh's defense makes every repetition a valuable one. Saleh and multiple players have referred to the free safety position as the "eraser." In other words, it's the free safety's duty to wipe away any mistakes that might be made elsewhere in coverage.

However, if there's a silver lining to be found in Ward's injury, it's that the 49ers can now focus on developing depth at a position where none has really been evident.

Up first is Jaquiski Tartt, a third-year pro who has previously looked to be better suited to playing strong safety but is considered the team's third safety, regardless of whom he must replace. During Friday's practice, Tartt took all of the reps with the first-team defense.

"I'm always ready to go," Tartt said. "I approach the game as if I'm a starter, so if anybody goes down, I'm ready to go. In the spring, I usually look at both [safety and corner] and study both just in case stuff like this happens. If somebody goes down, I'll be ready."

Tartt has appeared in 30 games in his first two seasons, with 14 starts. The 49ers have him down for 127 tackles with three sacks, an interception, six passes defended and a forced fumble in those games. Listed at 6-foot-1, 221 pounds, Tartt made it a point to get leaner in the offseason so as to have a bit more versatility in the defense.

"I think he's come a long way since we first got here," Shanahan said. "He's gotten into great shape. He did that throughout Phase 1, 2 and 3. He came back yesterday even in better shape. I'm really excited about him. He's a guy who can run and hit, and we've got to see how he plays in the system and how fast he can play once he gets out there."

In his previous two seasons, Tartt was big enough that he was even used as a linebacker in sub packages. But the No. 1 rule for a free safety in Saleh's scheme led him to believe he needed to get lighter and faster.

"Don't get beat deep and you have to be the quarterback back there," Tartt said. "You have to be the eyes for the defense and make sure you're being vocal, calling out the personnel and making sure you are getting everybody set."

During the offseason program, Tartt said he spent most of his time at strong safety but also got occasional reps at free safety. In Saleh's scheme, the free safety often is left on his own on the back end, while the strong safety spends more time near the line of scrimmage. Those jobs would seem to require a different mentality, though Tartt said it's not a major change.

"I would say it's the same, in a certain way," Tartt said. "You're at free safety, but if you see the run, you can go and attack the run. Strong safety, you are in the box and you are able to just be physical all day."