Theofanous also represents Blue Jackets goaltender Sergei Bobrovsky, a fellow pending unrestricted free agent.
On Friday morning, The Athletic's Aaron Portzline reported that Panarin fired Milstein earlier this week.
Panarin is set to become a UFA on July 1 and told the Blue Jackets he won't discuss his future with the club until the offseason. That uncertainty's made the dynamic forward one of the most attractive rental candidates ahead of the Feb. 25 trade deadline.
Theofanous is Panarin's third agent since January 2017. He switched from Tom Lynn to Milstein during his second contract negotiations with the Chicago Blackhawks.
Blue Jackets management reportedly didn't view Milstein as a barrier to extending Panarin, Portzline added.