The bad news for McDonald is he'll miss out on valuable playing time over the next eight weeks as he goes on injured reserve/designated to return.
From a Rams perspective, losing McDonald hurts not only in the sense that it's likely to slow his development, but it also thins an already light position.
The Rams entered training camp with rookie McDonald and Darian Stewart as the projected starters at safety. The duo changed when Stewart suffered a thigh injury in the second preseason game and Rodney McLeod took over. McLeod has handled the job opposite McDonald since but now the Rams will undergo another shakeup at a shaky position.
After McDonald left Thursday's game, Matt Giordano filled in, but the mix is made even more complicated by a hamstring injury to cornerback Cortland Finnegan. When Finnegan departed the loss to the Niners, the Rams had to put in their alternate looks in sub-packages. That meant McLeod shifted to the nickel corner role with Giordano coming in alongside McDonald.
With McDonald out, it means putting Stewart in alongside Giordano with McLeod in the nickel corner role if Finnegan is unable to play. That's just one permutation of the many moving parts the Rams will have to work out over the next 10 days for personnel groupings beyond the base defense.
Frankly, the safety position has not been a strength of the defense in the first four weeks, and losing one of the more promising prospects on the back end at this point can't be viewed as anything but a blow. That's not to say McDonald has been playing at an elite, Pro Bowl level, but he figures prominently into the team's long term plans, and every rep is valuable at this early stage in his career.
Fisher said Finnegan and linebacker Will Witherspoon have hamstring injuries and are day to day.