TAMPA, Fla. -- Buccaneers Pro Bowl linebacker Kwon Alexander, who left Sunday's overtime victory over the Cleveland Browns just before halftime with a right knee injury, is believed to have torn the anterior cruciate ligament in his knee, a source confirmed to ESPN.

He'll undergo an MRI on Monday to confirm the diagnosis, but the team fears that he is out for the season.

Alexander suffered the injury with 18 second left in the first half. He was blitzing quarterback Baker Mayfield and planted his foot, going down without contact. Players on both teams immediately knelt down. Alexander was able to leave the field under his own power.

NFL Network was first to report the news.

"He said to us before we left out there for halftime, he was like, 'Never take anything for granted,'" Bucs quarterback Jameis Winston said. "We see the passion he plays with. We see the heart he puts in the locker room. We are going to have his back just like we had his back today."

"I love Kwon. We're going to miss him and I know he is going to make an amazing comeback. That's just the type of person that he is and his work ethic has been strong since he's been here."

The timing couldn't be worse for Alexander, who is set to become a free agent next season, and has been trying to negotiate a new contract.

The Bucs are now razor-thin at linebacker. Rookie Jack Cichy, who stepped into Adarius Taylor's strong side spot (Taylor slid over to Alexander's middle spot), also left with a knee injury and did not return. Last year's starting strong side linebacker, Kendell Beckwith, is still recovering from ankle surgery after an offseason car accident.

The team did re-sign linebacker Devante Bond this week to provide more depth. Bond had been waived after suffering a foot injury during the preseason and received an injury settlement.