For those of you confused by this comment: I believe Manfred assumes Lucius suspected that Hermione was replaced by a polyjuiced Bellatrix Black. Lucius implies that he believes Harry to be a de-powered Voldemort in their discussion at the train station, and also believes Harry to be behind the rescue of Bellatrix from Azkaban. If you rescued your powerful minion, you would want to keep her close about you for your own protection and to accomplish tasks beyond your magical abilities. Hermione Granger is known to associate with Harry Potter, so she would be the ideal candidate for someone to replace with Bellatrix.
For those of you confused by this comment: I believe Manfred assumes Lucius suspected that Hermione was replaced by a polyjuiced Bellatrix Black. Lucius implies that he believes Harry to be a de-powered Voldemort in their discussion at the train station, and also believes Harry to be behind the rescue of Bellatrix from Azkaban. If you rescued your powerful minion, you would want to keep her close about you for your own protection and to accomplish tasks beyond your magical abilities. Hermione Granger is known to associate with Harry Potter, so she would be the ideal candidate for someone to replace with Bellatrix.