The team ranked 28th in rushing in 2014, earning just 88.9 yards per game. They then cut Reggie Bush in the offseason, just a year removed from a 1,006-yard rushing and 506-yard receiving season.
Offensive coordinator Joe Lombardi recently admitted that questions remain about the Lions backfield heading into 2015.
"I think it is a work in progress," Lombardi said, per the team's official website. "Joique (Bell) we love. He's a tough guy. Runs hard. Just keeping him healthy will be key.
"I'm very excited for Theo (Riddick) and have been since I first got here. I think we're going to see great things from him."
Bell has shown an ability to be a between-the-tackles runner who can excel in the passing attack. However, he is coming off a 223-carry season and averaged just 3.9 yards per tote each of the past two seasons. He needs a sidekick.
Riddick is a complete wildcard in the running game (29 career carries for 2.6 yards per), but adds a shifty weapon in the passing attack.