The starters for every team are going to naturally be better than the backups. That is how sports work. In an ideal world though, if a starter goes down the talent gap between the first string and second string players would be almost unnoticeable.
In 2018 BYU Football was not in that ideal world.
When Matt Hadley went down against Utah, the Cougar’s running game became non existent. What Corbin Kaufusi did against the Utes was incredible, but the reality is, it is pretty sad when the talent gap is so large that a Kaufusi needing three surgeries is more effective than a healthy back up player.