Kiser suffered the injury Saturday in a preseason loss to the Dallas Cowboys at Aloha Stadium in Honolulu.
A second-year pro, Kiser was a projected starter at inside linebacker alongside Cory Littleton. Fifth-year pro Bryce Hager, who has contributed mostly on special teams the past four seasons, is expected to fill the role.
"He's capable of being a starting player in this league," Rams coach Sean McVay said about Hager, before the results of Kiser's MRI were announced. "He's definitely demonstrated that when given the opportunity, he's capitalized on those chances."
In four seasons, Hager has three pass deflections, a forced fumble and a fumble recovery.
McVay has opted to sit most of his starters throughout the preseason because of the potential for injuries. He even went so far as to leave about 20 starters in Los Angeles rather than have bring them to Hawaii to lessen the travel burden. But Kiser, a first-year starter, and defensive tackle Sebastian Joseph-Day, also a projected first-year starter, have participated in preseason games because of their lack of experience and to solidify their roles as starters.
"The reason that we thought it was important for those guys to play is because they are still younger guys that haven't had the accumulation of work," McVay said. "Unfortunately, whether it is in preseason games or regular season, even in practice, sometimes these injuries are inevitable and you can't always avoid it."