I have Gurley, Malcolm Brown and Kelly and have been closely watching this and all signs lead to Brown being 1st up if Gurley goes down. Brown has gotten the first team reps when it mattered but kelly has has a good camp. From what I have read it may come down to pass pro where Brown has a clear step up on the rookie.
Brown is only on a 1 year deal so it would seem likely they use brown for that year and after a year of seasoning then Kelly will get that chance to take the #2.
For what its worth Brown ran very well last year when he was healthy with 4 games of 10+ touches and 45+ yards down the stretch keeping Gurley fresh for the playoffs.
I think Kelly is more talented than Brown but Brown is the incumbent with more experience who looked good when he touched the ball last year. Oddly enough, Brown suffered a high-grade knee sprain on a handful of touches last year which made me question how valuable a fantasy handcuff he could actually be. It could just be fluky but after holding him in deeper leagues as a high-upside stash I kind of just rolled my eyes when I heard the news.
Depending on when the Gurley injury occurred, I think you'd see a RBBC that slowly skewed more towards Kelly as the season progressed. Regardless, Gurley saw 87 targets last year so in ppr I think Kelly, the more proficient pass catcher (averaged a 15% target share in his college offense), would be more valuable in the short and long-term.
I'm really high on Kelly, though I do admit that it's premature to assume that he'd just slide into a bell cow role in the event of the Gurley injury. But I do think that if you're in the market for handcuffs this is generally a situation worth targeting. I don't wish injury upon Gurley, I'm not an #######. But he does have the most carries of any RB since 2015 (800) and he runs behind a good OL on a good offense with a good play caller who knows how to maximize talent. You also generally want to target handcuffs behind guys who touch the ball 25 times a game.