Schumacher claimed a podium during his three year comeback but Brawn believes Rosberg's pace relative to Lewis Hamilton provides a new insight into Schumacher's time with Mercedes.
"Lewis brings a great reference to the team and is a great driver, someone whose raw ability you know is at the very top," Brawn told Autosport.
"I think it was a little unfortunate with Michael because we didn't have these references. Michael had a tough time sometimes with Nico. Was that because Michael wasn't as good as he was? Or because Nico was better than people gave him credit for?
"I think everyone recognises now that Nico was better than people gave him credit for."
Rosberg is sixth in the 2013 championship, having claimed three pole positions and two victories.