I don't think it's about a backup playing as well as the starter. I think what matters is that when you plug a backup in to replace and injured starter, can they play well enough that the unit can maintain the same level of play as a unit. That's why a couple of injuries can be managed, but when you've got multiple starters missing in a unit it becomes a negative force multiplier that drags the entire unit down.
For instance, the offensive has four basic moving parts. There's the line, the QB, the backfield and the eligible receivers on the line. If you're just missing one lineman, you can work with that. If you're also missing one eligible receiver, it's more difficult. If you lose another receiver and another lineman and the QB and a backfield player, it's going to be nearly impossible to have the four moving parts all work in proper sync consistently.
When you have multiple starters out on one unit, it increases the likelihood of error and one error is all it takes to kill a play. It takes exceptional coaching to work around that. I don't know if the current staff will be exceptional, but I know last years wasn't. They had some really good coaches, but the coaching staff as a whole unit was less than exceptional. It wasn't less than zero, (hey...red), but it wasn't exceptional.