They're spending 2 Destiny Points. Let them use it after they take damage if they want.
If the effect only lasted for that one round I'd tend to agree however since it lasts for three full rounds I think it's appropriate to have the PC plan ahead and activate it before they take a hit. It's a powerful effect and three rounds is a sizeable chunk of combat.
I do see your point and understand where you're coming from.
However, I would still let the PC activate Unmatched Protection after they took damage and here is why: anything you spend Destiny Points on should have a guaranteed effect on the game. If you activated this ability without knowing that you truly need it, you risk it simply being wasted.
After all, it's entirely possible that the enemies fail to hit the PC while the talent is active through no fault or malicious intention of the GM. In my mind, whenever the PCs put something so valuable as 2 Destiny Points to do something, they should get something concrete out of that.