Carlos Hyde, expected to become the San Francisco 49ers' lead running back in the wake of franchise rushing leader Frank Gore's free-agent departure, will begin training camp on the active/non-football injury (NFI) list.

Hyde is one of four players placed on the list Tuesday by the Niners; the others are second-year outside linebacker Aaron Lynch, rookie right tackle Trent Brown and rookie receiver DeAndre Smelter.

Center Daniel Kilgore, who broke his left ankle at the Denver Broncos in Week 7 last season, was placed on the active/physically unable to perform list.

While Lynch was rehabbing a hamstring issue in the offseason training program and Smelter was drafted after tearing an ACL in December, Brown was getting reps with the first-team offense and did not appear to get injured during the offseason program, and the inclusion of Hyde on the NFI list is somewhat of a surprise.

Carlos Hyde, expected to assume the lead running back role for San Francisco, has opened training camp on the active/non-football injury list. Brad Penner/USA TODAY Sports

In OTAs, Niners rookie coach Jim Tomsula explained Hyde's absence as "nothing serious" at the time, though it was believed to be a calf issue.

"Carlos is -- look, we're not in a hurry with him right now," Tomsula said on May 29. "It's nothing serious, but let's be at 100 percent when we get to training camp. He's in all the meetings and he's here and he's working out and he's doing his stuff. It's just more on the cautious side of just making sure we've got everything healed, and then we move forward.

"It's not even a hamstring; it's a muscle pull. You talk to our medical people -- (at a) different time of the year, Carlos would be pushing through. But why? That's my whole point on everything. Why do that now and take any kind of chances on things? Let's just get it healed and get going."

Hyde, who averaged 4.0 yards on 83 carries and tied Gore for the team lead with four rushing touchdowns last season, sent a series of cryptic tweets this month, including one in which he posted only the word "hurt."

The 49ers signed veteran running back Reggie Bush as a free agent, drafted Mike Davis out of South Carolina in the fifth round, and welcomed back Kendall Hunter from a torn ACL that cost him last season. The team opens training camp with its first practice Saturday.

And while Hyde, Kilgore, Lynch, Brown and Smelter still count against the team's 90-man roster, which is at its limit, they can be activated at any time before final cuts at the end of camp.