SANTA CLARA — San Francisco defensive tackle Kalia Davis will need to undergo surgery on his knee after getting hurt in the exhibition opener in the latest injury to hit the 49ers.

San Francisco was missing more than a dozen players because of injuries at practice on Monday, leading coach Kyle Shanahan to cancel a planned joint practice later this week against the New Orleans Saints.

“It was tough because we like to do it,” Shanahan said Monday. “In the long run, I waited so long to make the decision because of how much we enjoy doing it. But when we realized we were just doing it because we were enjoying to do it and trying to change the monotony it didn’t make it worth it.”

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Davis and Thomas Face Extended Absences

While most of the injured players are dealing with minor injuries that should be healed by the season opener on Sept. 9, Davis will miss significantly more time. Shanahan said he expected the third-year defensive tackle to miss about half of the regular season after getting hurt Saturday night. Cornerback Ambry Thomas also broke his forearm in that game and will miss about half of the season as well.

Davis had one sack and two tackles for loss against Tennessee on Saturday, showing the flashes the Niners had been hoping for after Davis played sparingly his first two seasons while recovering from a knee injury in college.

“I thought he played one of his best games,” Shanahan said. “It was really unfortunate. He played awesome through the whole game, and that is why we, he and us were surprised he had such a tough injury.”

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Joint Practices Canceled Due to Injuries

While Thomas and Davis have the only injuries that are projected to last into the season, several other players, including Christian McCaffrey, Elijah Mitchell, Ricky Pearsall, Isaac Yiadom, Jon Feliciano and Spencer Burford, are sidelined right now, leading to the decision to call off the practices with the Saints.

The 49ers like the joint practices because it allows his starters to get more work in a structured environment than they do in exhibition games but decided it was best to pass on it this year because of health.

“I was very disappointed,” cornerback Deommodore Lenoir said. “We’ve been competing with each other for so long. I feel like being able to actually unleash that on another team would have been great for two days and then play some in a game. Just get a little scrappy out there.”

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Contract Situations and New Additions

San Francisco is still missing two other key players at practice with receiver Brandon Aiyuk conducting a “hold in” and left tackle Trent Williams holding out in search of new contracts.

Aiyuk is attending meetings but hasn’t practiced or worked out with the team but Shanahan believes he will be ready for the season if the contract situation is resolved.

“Brandon knows what he has to do for this year to get ready to play,” Shanahan said. “I’m feeling he’s finding a way to stay in shape and do those things, but there’s a football-shape element he’s eager to get back to. Hopefully he’ll get to that soon. Brandon knows what he has to do to be in shape for the season and I fell confident he’s doing that.”

The Niners brought in some help on Monday when they signed veteran receiver Robbie Chosen to a one-year contract and waived receiver Frank Darby with an injury designation.

The 31-year-old Chosen, who was known as Robby Anderson when he entered the league in 2016, had 4,155 yards receiving in his first five seasons in the NFL with the New York Jets and Carolina but has been far less productive the past three seasons.

Anderson played nine games last season for Miami, which runs a similar offense to San Francisco, with just four catches for 126 yards and one TD.

Safety Talanoa Hufanga could also return soon from a torn ACL suffered last November. Hufanga will meet with doctors on Thursday to determine if he’s ready to return to practice.