Bhaktisiddhanta Sarasvati Thakura Prabhupada told that only a practicing Acarya who is in the Paramahamsa stage can act as an effective Acarya. It is only possible if the Acarya has direct feelings (realizations) about Shabda-Brahma and Para-Barhma. Then he can act as an effective Acarya. Otherwise, no one can act as an Acarya.
