Brown is a very solid backup RB but I think the "huge loss" comment is more about complimenting his effort, teamwork, etc. than anything. Brown has played 123 snaps this season (10/game) on offense. Of course, Davis and Kelly have combined for zero snaps, so...
However, Kelly has been active for 8 games and played 111 snaps on special teams so far vs. zero for Kelly. So there's that. I am guessing both will be active going forward but which receives actual snaps on offense and touches will be interesting to watch. I do think Gurley should continue to get spelled 10-12 snaps/game.