If he is already listed IR by team, and he is on your roster, tag down position to IR (it will show the availabilty). Then you can pick up another player in the meantime. If he is FA, then you need to drop a player, pick up the IR. Your player will be on waivers, you should plan on another FA player to replace the drop. To activate from IR, you need to drop a player to make room on roster for the activation.