SANTA CLARA – Not only did the 49ers lose linebacker Kwon Alexander to a season-ending injury in Week 9, but linebacker Fred Warner did not play well that night against the Arizona Cardinals.

Warner responded Monday night with what 49ers defensive coordinator Robert Saleh described as the best game of his two-year NFL career.

“I thought he was just on it,” Saleh said. “He played violent.”

Warner recorded 10 tackles, including the first two sacks of his career, and forced a fumble that DeForest Buckner scooped up and returned for a touchdown. He became the first player with that stat line since Chicago defensive lineman Akiem Hicks on Dec. 4, 2016.

Warner did it all for the 49ers’ defense in the club’s 27-24 overtime loss to the Seattle Seahawks on Monday night at Levi's Stadium while taking on more responsibility with rookie Dre Greenlaw taking over for Alexander.

“That was really cool to watch, just him pick up the slack that we lost from Kwon and play with that much passion,” 49ers defensive end Nick Bosa said. “It’s fun to have him on the field.

“It was usually him calling the plays and Kwon being the energy guy. And now it’s him doing both. He calls the play. He gets us lined up. And he brings the energy, so he’s really important.”

Nick Bosa said #49ers second-year LB Fred Warner is the last player out of the team’s facility every day, including on Tuesday. Bosa said, "I was at home, waiting for him to get on Xbox with me, and he didn’t leave here until 5, and it was our off day." — Matt Maiocco (@MaioccoNBCS) November 14, 2019

Warner displayed his ability to rush the passer, which is a new look for the 49ers’ defense. Saleh said he believes the threat of Warner coming on blitzes will prove beneficial to the team’s defensive linemen.

“The most important thing for us is, by them having to account for him, especially using a lineman, is now you’ve got your four D-linemen in one-on-one situations, our four horses,” Saleh said. “If they’re in one-on-one situations, you’re pretty confident that one of them is going to win. They’ve got to bring in extra people to protect and now it’s are we blitzing or are we rushing four?

“It’s just a cat-and-mouse game. When you have to account for him, you’re freeing up your four horses in the one-on-one situations.”

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Warner and the 49ers return to action against the Cardinals on Sunday at Levi's Stadium. The 49ers' defense had its worst game on Oct. 31 in a 28-25 victory over Arizona, a game in which Warner had two missed tackles. Saleh said he expects more performances from Warner like the one he had against the Seahawks.

"He had a very good game and now it goes back to consistency being the truest measure of performance. He’s got to go stack it up and do it again," Saleh said.