Any time a starter is injured and replaced by a backup, a drop-off in talent will occur. There’s usually a reason the starter is starting and the backup is second on the depth chart. Few teams (if any) employ “great” backups. If the backups were great players, they would be starting.
With that being said, there is still a huge discrepancy in the talent level of backups across the league. Some backups are capable of closely matching the production of the starters ahead of them, while others cannot even come remotely close. Others are future starters only masked as backups simply because their time has yet to arrive.