How do you install a Downloaded update to a PS4 gamed Owned on Disk?

My son owns the disk version of Injustice 2 and recently bought and downloaded the update with new characters from the Playstation Store. The Downloaded fine and we can see them in the on this PS4, but they won't install or aren't available in the game. Are we missing a Step? Is this because he has the game on Disk? Do we need to download a different version of this update/Add On that's compatible with the Disk version of the game?