Rule 614.4 · Replacement Effects

Replacement effects must exist before the appropriate event occurs—they can’t “go back in time” and change something that’s already happened. Spells or abilities that generate these effects are often cast or activated in response to whatever would produce the event and thus resolve before that event would occur.

Example: A player can activate an ability to regenerate a creature in response to a spell that would destroy it. Once the spell resolves, though, it’s too late to regenerate the creature.