Mourinho, who manages the goalkeeper at Manchester United, insists that the Spaniard does not make such mistakes at Old Trafford, but concedes that even the best custodians can err from time to time.
"It's my boy, it hurts me to say but he knows it's a bad mistake," Mourinho told RT after the 3-3 draw.
"It's a mistake he doesn't make with us, United - player of the season, fantastic performances. But that happens to the best.
"The good thing with the best is that in the next match he will be there - not afraid, confident to go and ready to help his team. It is a goal with David's face."
De Gea has won 30 caps for Spain, keeping 15 clean sheets.