Hernandez is still contracted to United, but will spend the 2014-15 campaign on loan with Los Blancos.
"He's a great person. I think he'll do really well there," De Gea told El Larguero.
"I would have liked him to stay, but he was keen to leave and get regular games. I think he'll score lots of goals at Real Madrid."
De Gea is currently on international duty with Spain ahead of the opening round of Euro 2016 qualifying fixtures.