The injury occurred when receiver Kenny Britt stepped on Gaines' foot as the two got tangled during a practice drill. The result was a Lisfranc injury for Gaines that required surgery and will keep him from returning this season.
The St. Louis Post-Dispatch first reported the news of Gaines' surgery.
Gaines emerged as one of the team's starting corners in 2014 after presumed starter Trumaine Johnson suffered a knee injury in the team's third preseason game. Gaines played well enough in Johnson's absence to keep the job when Johnson returned in Week 9.
After posting 70 tackles, two interceptions, two fumble recoveries and eight pass breakups in 15 games last year, Gaines was the odds-on favorite to win a competition with Johnson for the outside starting spot this year.
That competition never really took form, though, as Gaines suffered the foot injury less than a week into camp. The Rams had maintained that Gaines was still undergoing tests as recently as Saturday night after Gaines had visited foot specialist Dr. Robert Anderson in Charlotte, North Carolina.
Without Gaines, Johnson figures to move back into his starting role with Lamarcus Joyner handling the slot in the nickel package.