Answer
What is the difficulty in simply calling someone “Prabhu”? There is no difficulty. In fact, it strengthens the right understanding. Because he is a dāsa of Krishna, he is my Prabhu—this becomes a double reinforcement of the proper mood.
But more important than just saying it is feeling it. It should not be mechanical. You don’t have to merely call—you have to genuinely feel that consciousness within.
So when you say, “Yes, Prabhu,” it should come from that inner understanding and respect, not just as a formality.