Rams coach Jeff Fisher said Friday that McDonald's fractured right leg was not displaced and will not require surgery, which puts him on a trajectory to return before the end of the season.
Given that diagnosis, the Rams used their lone injured reserve with the designation to return tag on McDonald, allowing them to replace him on the active roster while not officially ending his season.
"It times up with the doctor's estimation of when he'll be back, so it's not season-ending," Fisher said. "So that's, in a lot of ways, good news for us."
The designated to return provision began last season, and the Rams used it on center Scott Wells.
Under the rule, which requires the player to sit out eight games, McDonald will have to miss the next six weeks before he can return to practice. From there, he has two weeks to practice before a decision must be made.
Losing McDonald is another blow to an already struggling secondary. In four games, the rookie out of USC was fifth on the team in tackles with 18 and was a key contributor on special teams.
"He was playing well," Fisher said. "In addition to those things defensively, you know, he was close to blocking punts and he was playing well for us on special teams."
After McDonald exited Thursday's game, the Rams used veteran Matt Giordano in his place. Giordano and Darian Stewart figure to get opportunities with McDonald out, and the team promoted rookie Cody Davis from the practice squad for depth.