Adam Schefter and Louis Riddick react to the 49ers' decision to sign Kwon Alexander to a four-year contract. (0:55)

Former Tampa Bay Buccaneers linebacker Kwon Alexander plans to sign a four-year, $54 million deal that includes $27 million guaranteed with the San Francisco 49ers, according to ESPN and multiple reports.

Before Alexander sustained a torn ACL on Oct. 21 last season, the Buccaneers were willing to offer him as much as $10 million annually, a source familiar with the negotiations told ESPN.

But the feeling at the time was that Alexander, 24, who led the NFL with 108 solo tackles in 2016, was seeking closer to Luke Kuechly's annual salary of $12.4 million and an agreement could not be reached. Now Alexander has topped Kuechly with a deal that averages $13.5 million per season.

After re-signing left tackle Donovan Smith last week, the Bucs had less than $3.5 million in salary cap space.

Even after the injury, the Bucs expressed a desire to retain Alexander, whom general manager Jason Licht called the "heartbeat of the defense" -- but at the right price.

Sources say Alexander's recovery is right on schedule. He has been under the care of Dr. James Andrews and has resumed weightlifting -- including squatting -- and is now jogging.

Alexander's strength is being active up front and in the backfield. He's particularly physical against the run. At times, he has encountered challenges dropping into coverage.

Alexander is at his best when he can be active up front and getting into the backfield. As a rookie in 2015, the fourth-round draft pick earned the starting middle linebacker job over veteran Bruce Carter, immediately becoming one of the Bucs' most physical players.

He was selected to the Pro Bowl in 2017 and has 380 tackles, seven sacks, six forced fumbles and six interceptions in 46 games since being selected by Tampa Bay in the fourth round (124th overall) of the 2015 draft.

ESPN's Jenna Laine contributed to this report.