Ok, your CO-OP bike should be pretty new as they changed the name on them a couple of years ago.

About 4 years ago Shimano introduced what they call "Flat Mount" disc brakes. In particular it moves the rear brake to a spot inside the rear triangle.

https://road.cc/content/buyers-guide...ut-disc-brakes

I'd get the correct brake, but I'm not quite sure which one.

Can you see any numbers on the old brake once you have it off?