Pro Bowl linebacker Kwon Alexander, who left the Tampa Bay Buccaneers' overtime victory over the Cleveland Browns just before halftime with a left knee injury, suffered a torn anterior cruciate ligament and will miss the remainder of the season, Bucs coach Dirk Koetter announced Monday.

Alexander underwent an MRI on Monday to confirm the diagnosis. He was placed on injured reserve.

"It's unfortunate. Kwon was playing so well, and he's the heart and soul of our defense," Bucs coach Dirk Koetter said Monday. "It's very unfortunate. Our locker room at halftime was extremely emotional last night. Kwon came out of the training room right around the time we were getting ready to go back onto the field. It was emotional."

Alexander suffered the injury with 18 seconds left in the first half of the Bucs' 26-23 victory Sunday. 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.

"He said to us before we left out there for halftime, he was like, 'Never take anything for granted,'" Bucs quarterback Jameis Winston said Sunday. "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.

Left tackle Donovan Smith, who was part of Alexander's 2015 draft class along with Winston and Ali Marpet, said, "He's like my brother. It's really tough. I'm at a loss for words, honestly. He's such a great guy, a great player that's huge for our team."

The Bucs are now razor-thin at linebacker. Rookie Jack Cichy, who stepped into Adarius Taylor's strongside spot (Taylor slid over to Alexander's middle spot), also suffered a torn ACL in his left knee and is out for the season.

Last year's starting strongside 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.

To help fill the void, the Bucs re-signed linebacker and former "Hard Knocks" star Riley Bullough, who had suffered a foot injury during the preseason and reached an injury settlement with the club in September.