Namor(Imperius Rex) Knocked Out Sidekicks

If Namor(Imperius Rex) and two sidekicks are attacking and the sidekicks get knocked out, do they go to the transition zone until opp. turn? -or- do the sidekick go straight to the used pile, so that Namor's ability can target those two side kicks. (there are no dice in the used pile prior to this happening)