On Monday, 49ers head coach Kyle Shanahan said cornerback Richard Sherman would be sidelined likely a “couple of weeks” with a Grade 2 hamstring strain he sustained in Sunday’s win at New Orleans.

On Thursday, Sherman offered a different outlook. He maintained he could play Sunday against the Falcons at Levi’s Stadium, if he weren’t being held out because of his partially torn hamstring.

“I could play in this game if we had to have it,” Sherman said. “Kyle and them are more cautious than I am. … I told them I just needed a couple of days. I needed a couple of days and I feel great today already. (I ran) 19 miles per hour, 20 miles per hour today. That’s really all you need to play.”

Sherman said he expects to return to play against the Rams on Dec. 21. And his ability to play through injuries throughout his nine-year career suggests he’ll be able to do just that.

Sherman said his current hamstring strain isn’t as severe as the hamstring injury he suffered in the 2017 season opener, when he was with the Seahawks, and he didn’t miss a snap in that game against Green Bay. Sherman said he also played through a Grade 2 hamstring strain he suffered last year in a Week 2 win against the Lions.

Sherman, who began his career by playing in 115 straight games, played the majority of the 2016 season with a sprained medial collateral ligament.

“I’ve had these (hamstring injuries) before,” Sherman said. “I’ve played through them. This will be the first one I’m not being able to (choose) to play through.”

Dr. Kenton Fibel, the team physician for the NHL’s Anaheim Ducks, said a Grade 2 hamstring injury often can sideline athletes for about a month. Fibel spoke Thursday before Sherman addressed reporters.

“Some of them can come back a little sooner,” said Fibel, a primary-care sports-medicine specialist at Cedars-Sinai Kerlan-Jobe Institute in Los Angeles. “So three weeks may be possible for him, but that would certainly be on the earlier side of things. There’s always the risk of re-aggravation in trying to get a player back a little earlier for a game.

“So that’s certainly something that I’m sure he’ll be discussing with the team physician, just looking at his future and the team’s future and the likelihood of being in the playoffs.”

Celek on IR: The 49ers placed tight end Garrett Celek on injured reserve with a back injury after Celek returned from offseason back surgery to play 61 snaps in five games this season.

Celek, 31, the team’s second-longest-tenured player, appeared on the injury report Wednesday with a back issue. He has had two back surgeries in his eight-year career.

The 49ers promoted undrafted rookie tight end Daniel Helm from the practice squad to assume Celek’s roster spot.

Eric Branch covers the 49ers for The San Francisco Chronicle.