They can suffer from a condition with identical mechanical game effects.
This is really the way to go in my opinion. People don't like some conditions based on their names and the visual representations they provide. My players hate that immobilize creatures can stand from prone for example. Or that a slowed creature can Charge.
Don't think of the ooze being knocked prone as much as being debilitated by the attack in such a way that it requires a move action in order to properly stabilize and attack at peak effectiveness once again.