RB is not a big deal. You got 3 quality backs and a decent stable of developmental backs with guys like Perry and Johnson. If Ajayi goes down of course you take a hit, but you could say that about every single position on the team. RB is stronger to handle that than most of the other positions.
Mangold is a solid veteran but he is on the decline and he is looking for a contender and a starting opportunity. All while lessening the tear on his body by not being on a team during camp. The name is sexier than the product IMO. Obviously if Pouncey has a setback they could look in that direction with the Tannenbaum connection but still it is up to Mangold to be satisfied coming here.
For me, with Asiata struggling some, the main issue continues to be guard. Need to have some luck there and have a quality third guard available in case anything happens to our starters. You can have Urbik be the active gameday interior who can come in if a guy goes down at all 3 positions . . . but he needs to be Pouncey insurance and we need a guy who is ready to step in the following week if Bushrod/Larsen go down for an extended period.