Darnell Dockett was supposed to fill void left by Justin Smith

Darnell Dockett was expected to fill the void, both on the field and in the locker room, left by the retirement of Justin Smith.

Dockett, like Smith, began playing in the NFL in the first George W. Bush administration and, like Smith in his prime, was a pass-rushing menace.

But Dockett, like the retired Smith now, is a former member of the San Francisco 49ers, having been a surprise cut Friday, per ESPN and several reports, as the team pares its roster to 53 players from 75 before Saturday’s 4 p.m. ET deadline.

What remains on the Niners’ defensive line, then, is a somewhat unproven commodity when it comes to rushing the passer. Dockett, a three-time Pro Bowl selection who was attempting to come back from missing all of last season with a torn ACL, has 40.5 career sacks in 10 seasons, including nine in 2007. The rest of the defensive linemen have 12 sacks in a combined 17 seasons. Glenn Dorsey, a nose tackle by trade now starting at left defensive end in the Niners’ 3-4 scheme, has six of those sacks.

Financially, Dockett was reportedly guaranteed $2 million of his two-year, $7.5 million contract and was being groomed to become the pass-rushing specialist from the left side in the Niners’ nickel package.

But with Dockett gone, who fills that role?

That likely becomes the player he was expected to mentor – first-round draft pick Arik Armstead, who was taken No. 17 overall out of Oregon.

Dockett, 34, did not play in Thursday night’s preseason finale against the San Diego Chargers, dealing with what coach Jim Tomsula described as a contusion and cracked cartilage in the ribs.

Armstead played a team-high 58 snaps Thursday, perhaps portending the future.

“I got a lot of reps, which is good,” Armstead said after the game, per CSNBayArea.com. “A lot more experience. I wanted to get the playbook down and get more comfortable with that throughout camp and work on my technique, and I think I did a good job of that.

“It’s something to build on going into the regular season.”

And it’s something made possible by Dockett not panning out with the Niners.