Rams signed Alexander to a four-year, $2.68 million contract that includes a signing bonus of $464,260. Throughout training camp, Alexander competed for a job as a backup safety against Cody Davis, Christian Bryant, Lamarcus Joyner, and Matt Daniels. Head coach Jeff Fisher named him the backup strong safety to begin the regular season, behind T. J. McDonald. He made his professional regular season debut in the St. Louis Rams' Week 2 matchup at the Tampa Bay Buccaneers, but only appeared on special teams during their 19–17 victory. On November 16, 2014, Alexander recorded two solo tackles in the Rams' 22–7 win against the Denver Broncos in Week 11. He made his first career tackle during on a ten-yard kick return by Andre Caldwell in the fourth quarter. He finished his rookie season with four solo tackles in nine games and zero starts while solely appearing on special teams. 2015 During training camp, Alexander competed against Mark Barron and Cody Davis to be the Rams' primary backup strong safety. He was named the third string strong safety to begin the regular season. On September 27, 2015, Alexander sustained a groin injury in the third quarter of a 12–6 loss to the Pittsburgh Steelers. He left the game due to the injury and was inactive for the next two games (Weeks 4–5). On November 8, 2015, Alexander earned his first career start after starter T.J. McDonald sustained a foot injury. He recorded three solo tackles and made his first career sack during their 21–18 overtime loss at the Minnesota Vikings. Alexander made his first career sack on quarterback Shaun Hill and dropped him for an 11-yard loss in the fourth quarter. On December 9, 2015, the St. Louis Rams placed T.J. McDonald on injured reserve for the remainder of the season due to a shoulder injury and named Alexander his replacement at strong safety for the last four games. In Week 16, he collected a season-high eight combined tackles (four solo) during a 23–17 victory at the Seattle Seahawks. The following week, Alexander made a season-high seven solo tackles in the Rams' 19–16 loss at the San Francisco 49ers in Week 17. He finished the season with 39 combined tackles (30 solo), two sacks, a pass deflection, and a fumble recovery in 14 games and five starts. 2016 Alexander entered training camp in 2016 slated as a backup safety and competed for