Let's first just caveat that Kelley was playing against the ones, and Perine, the twos-
But upon further investigation, there is much to compare and contrast, and much to learn about how this all will more than likely shake out, so let's discuss...
Here's a video compiled of Kelley vs. Perine from week 2 (note- much watch on youtube):
On Fat Rob's very first run of the game, we can see his indecisiveness; that stretch play was blown up by their LB #50; note the backside 'bend' is more or less there, meaning with better vision, a quicker cut could have been made here instead of running right into Lavauo's back. That said, Lavauo was pushed backwards on the play... but he did manage to block/chip the penetrating LB. Actually Fat Rob tries to cut back late, but then doesn't commit and ends up getting gang tackled.
Im not going to comment on any other of his runs, because to me, the same sort of issues are on display throughout; indecision, and lack of burst. There were for sure some missed blocks as well, so it's not totally his fault, but he has to create where he can. Feel free to comment on other Fat Rob runs if you guys want. I don't think he did terrible- had some decent runs, and again, against the ones and the blocking is still coming together... but to me- love the guy- but he's a backup.
The last play I want to mention is- @ :27- positioning out of the backfield and the ability to pose a threat/keep defenses balanced in coverage is key. in the red zone, Kelley goes out into the flat and Cousins is in trouble; Moses eventually handles Matthews and Cousins spins out, but at that point the play has broken down. Fat Rob continues to 'back up' and move further away from Cousins, casing him to throw it in the ground. That was a chance for the RB to get something positive (in the red zone) with the check down and it was UGLY.
Now- Perine showed up, like I knew he would. proof is on the film- decisive, solid vision, finishes runs. Dual threat. Looks like a pro back.
One play in particular, @ 4:06 in the vid, a 3rd and 2, same LB for GB on the field (#50) that blew up first run of Fat Rob. Perine reads him and beats his angle, turning the corner and benefiting from solid downfield blocking.
Best run of the night though? at 4:34, 2nd and 4, this is vintage stuff!!! Sammy Perine sort of gets real choppy with his feet, cuts upfield and friggin PUNISHES #37. This is what you want from your RB- to be a physical presence that comes alive at the end of the game. Gonna see a lot of that this year.
I'm calling it right here- Perine is the starter by week 4. You wanna scrutinize the run game, gotta consider RB performance...it's not just the line.