As a junior in 2009, Foster started all 12 of the Huskies' games at one linebacker spot, finishing second on the team with 85 total tackles while leading the Pac-10 with six forced fumbles.

[4] Prior to the Senior Bowl, Foster was projected to be a third or fourth round pick by the majority of NFL draft experts and scouts.

On August 24, 2011, Foster was fined $20,000 for a hit he delivered to Chad Ochocinco during the Buccaneers' 31–14 loss to the New England Patriots in their second preseason game.

[14] Head coach Raheem Morris named Foster the starting middle linebacker to begin the regular season after he had an impressive showing during the preseason.

[15] He made his professional regular season debut in the Tampa Bay Buccaneers' season-opener against the Detroit Lions and recorded seven combined tackles in their 27–20 loss.

[16] In Week 14, he recorded six combined tackles, deflected a pass, and made his first career interception in the Buccaneers' 41–14 loss at the Jacksonville Jaguars.

Foster made his first career interception off a pass by Jaguars' quarterback Blaine Gabbert, that was originally intended for tight end Marcedes Lewis, in the endzone to secure a touchback during the third quarter.

[17] He finished his rookie season in 2011 with 84 combined tackles (58 solo), two pass deflections, two sacks, a forced fumble, and an interception in 16 games and 15 starts.

[21] On September 16, 2012, Foster collected a season-high 13 combined tackles (12 solo), a pass deflection, and an interception during a 41–34 loss at the New York Giants in Week 2.

[18] Foster returned as the starting middle linebacker to begin the regular season in 2013, alongside Lavonte David and Dekoda Watson.

[25] On September 15, 2013, Foster made seven combined tackles, a season-high two pass deflections, and returned an interception for his first career touchdown during a 16–14 loss against the New Orleans Saints.

Foster intercepted a pass by Saints' quarterback Drew Brees, which was originally intended for tight end Jimmy Graham, and returned it for an 85-yard touchdown in the fourth quarter.

[26] In Week 11, he recorded three combined tackles, broke up a pass, and returned an interception for a touchdown before exiting in the second quarter of the Buccaneers' 41–28 victory against the Atlanta Falcons after sustaining a concussion.

Foster intercepted a pass by Falcons' quarterback Matt Ryan, that was intended for fullback Patrick DiMarco, and returned it for a 37-yard touchdown in the second quarter.

Foster sustained a concussion after colliding with teammate Dashon Goldson and was inactive for the Buccaneers' Week 12 victory at the Detroit Lions.

[31] On September 14, 2014, Foster recorded two solo tackles before exiting in the second quarter of the Buccaneers' 17–14 loss to the Miami Dolphins after injuring his shoulder.

That’s a very valuable thing when you look at middle linebackers.”[31] Foster became an unrestricted free agent in 2015 and received interest from the Chicago Bears and San Francisco 49ers.

[40][12] Head coach Jay Gruden named Foster the backup inside linebacker to start the regular season, behind Perry Riley and Keenan Robinson.

[41] On December 7, 2015, Foster earned his first start of the season and recorded seven combined tackles during a 19–16 loss to the Dallas Cowboys on Monday Night Football.

[42] Foster started the last five games of the regular season (Weeks 13–17) after Perry Riley broke the fifth metatarsal in his foot and underwent surgery.

[53] He started in the Washington Redskins' season-opener against the Philadelphia Eagles and collected a season-high nine combined tackles and recovered a Carson Wentz fumble in the first quarter of their 30–17 loss.