PITTSBURGH, Pa. - The latest makeover on the Pittsburgh Steelers is accelerating as free agency begins.

The Steelers signed linebacker Arthur Moats to a three-year contract on Monday and released veteran defensive end Brett Keisel as the youth movement that's been underway for the last several seasons nears completion.

Moats, who turns 27 next weekend, had four sacks in his first season with the Steelers in 2014 after coming over from Buffalo. He added 24 tackles, two forced fumbles and a season-turning fumble recovery at Cincinnati on Dec. 7, feeding a furious rally that helped propel Pittsburgh to a 4-0 season-ending run and the AFC North title.

Moats filled in capably at both inside and outside linebacker last season for the AFC North champions. Pittsburgh is in serious need of help at outside linebacker: James Harrison and Jason Worilds are free agents, leaving Jarvis Jones as the only experienced outside linebacker on the roster when the new league year begins on Tuesday.

Keisel had one year remaining on his contract but his departure was anticipated after the two-time Super Bowl winner tore his triceps in a loss to New Orleans on Nov. 30. The Steelers declined to bring back Keisel last spring only to re-sign him at the end of training camp when his replacements struggled to stay healthy or make an impact. The 36-year-old, known across the league for the massive beard he grows — then shaves — for charity each season, had one sack and eight tackles in his final season with Pittsburgh.

"We have had conversations with Brett over the past couple of weeks during which we communicated our intentions to him," Steelers general manager Kevin Colbert. "Brett has played a major role in our success during his 13 years in Pittsburgh. We appreciate his efforts and we are grateful for what he helped us accomplish, including two Super Bowl championships. We will always consider him an all-time Pittsburgh Steeler."

Keisel follows linebacker James Harrison out the door. Harrison came out of a brief retirement to have 5 1-2 sacks last fall. He will remain in training and will explore free agency, but the 2008 NFL Defensive Player of the Year is not expected to return for a third act with the Steelers.

Troy Polamalu will almost certainly join Keisel and Harrison in the unemployment line. The eight-time Pro Bowler has two years remaining on his current deal, but saw his play dip noticeably in 2014. Colbert said Monday the team remains in communication with Polamalu about 2015 and beyond. Polamalu has a cap number of $8 million next season, but the Steelers can save a significant chunk of that money if they cut him with a post-June 1 designation.

Pittsburgh shored up depth in certain areas on Monday, agreeing to terms with backup tight end Matt Spaeth and tendering contracts to restricted free agent cornerback Antwon Blake, safety Robert Golden and fullback Will Johnson.

The Steelers and Spaeth agreed to a two-year deal. The 31-year-old Spaeth is used primarily as a blocker. He caught three passes for 46 yards and a touchdown in 15 games with Pittsburgh in 2014, including a 33-yard strike from Ben Roethlisberger in the fourth quarter of a victory against Baltimore on Nov. 2 that gave Roethlisberger an NFL-record six touchdown passes in consecutive games.

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