When you announce Collateral Damage, you put it on the stack, choose targets, activate mana abilities, then are required to pay all its costs. When you read the card you’ll find that you need to sacrifice a creature as part of that cost. This means that when Collateral Damage becomes “cast,” (which is just after you’ve finished paying all the costs) Monastery Mentor is no longer on the battlefield. No Mentor, no trigger. Any tokens that were created before this will of course get the prowess trigger and be bigger for the remainder of the turn.
So if you get the choice, it might be a better idea to chuck a token rather than the mentor itself. If you don’t have the choice (Monastery Mentor is about to be destroyed) then you might as well throw it in for some extra damage (even though you won’t get the extra token).
Bonus tip: If you were hoping to cast Collateral Damage and get the monk token in time to sacrifice it, you may have noticed by now that that’s also not possible, since you must sacrifice a creature before Monastery Mentor could even trigger to create a token.
Today’s Rules Tip written by Daniel Clarke