First, your prioirity should be to win Game 6 at all costs, if you have a chance to win Game 6. Therefore, it doesn't matter that you'd have to start Morton on short rest in Game 7 if you have a chance to win in Game 6 right in front of you. In that situation, with a lead, how could you pitch anyone other than McCullers? Who else has given you the confidence they can get it done?
And if you have to start Morton on short rest in Game 7, whooptie doo. It's the World Series Game 7, all hands will be on deck anyways. He probably would have pitched in Game 7 whether he or McCullers started anyways.