I think the decision about how this role should be filled should be made in the same way any other bullpen decision is made. Take into account the opposing lineup, how many batters the pitcher would be facing, who in the bullpen is fresh, who in the bullpen has or has not been pitching well recently, the game situation, the projected needs for the rest of that game and upcoming games, who is ready and available to be called up from St. Paul, etc. etc. etc.