Day and Marsh are welcome additions

Defensive coordinator Robert Saleh was giving thanks before Thursday.

On Wednesday, two days after the 49ers claimed defensive tackle Sheldon Day off waivers from Jacksonville, they claimed defensive end Cassius Marsh off waivers from New England.

Happy about those unexpected additions? “Heck, yeah,” Saleh said. Day, 23, and Marsh, 25, are fourth-round picks whose familiarity with the 49ers’ 4-3 scheme could allow them to play Sunday against the Seahawks.

The Jaguars also run a 4-3 and Day spent last year in Jacksonville with Saleh, who was the Jaguars’ linebackers coach. Saleh said the glut of productive linemen on Jacksonville’s No. 1 defense made Day expendable. The Texans, Lions and Cardinals reportedly put in claims for Day, who went to the 49ers because they had second waiver priority behind the Browns.

“He’s an NFL football player,” Saleh said. “… He’s got a tremendous skill set that we like, with his speed and quickness and his ability to penetrate off the ball.”

Marsh, who spent his first three seasons in Seattle’s 4-3 scheme, was traded Sept. 2 to the Patriots for fifth- and seventh-round picks. In nine games with New England, Marsh made one start and had 19 tackles and one sack.

In his first practice Thursday, Marsh was at the Leo position, Saleh said, which is reserved for the team’s best passrushers.

“When you watch his tape, he’s relentless,” Saleh said. “He does not stop. He’s the definition of all gas, no brakes. He does provide added juice off the edge.”

Meanwhile, the 49ers could get three defensive linemen who have been sidelined back Sunday: Solomon Thomas, Tank Carradine and Aaron Lynch.

Lynch (calf ), who has missed four games, is no longer on the injury report, and head coach Kyle Shanahan is hopeful Thomas (knee) will return from a two-game absence. Carradine is eligible to come off injured reserve after sustaining a high ankle sprain in Week 3.

Saleh said the unit’s improved health would mean some linemen would be healthy scratches Sunday. Safety spot: With rookie Adrian Colbert (thumb) unlikely to play Sunday, Saleh said Antone Exum, Dexter McCoil and Leon Hall are competing to assume his spot at free safety.

“It’s good competition and we’ll have it sorted out by Sunday,” Saleh said.