The Germany international will leave Allianz Arena on a free transfer at the end of June, with the defender deciding not to sign a new contract with the Bundesliga champions.
According to Struth, Sule, who has won four Bundesliga titles and the Champions League during his time at Bayern, has already decided his next club.
"The new club has already been decided. Niklas has made a decision and I would be surprised if he changed his mind", he told Sport1.
Sule has featured on 159 occasions for Bayern in all competitions since arriving from Hoffenheim, while he has been capped on 37 occasions by the German national team.