Fosu-Mensah was adjudged to have brought down Ross Barkley with Man United leading 1-0 at Wembley in the second half.
“I think giving away the penalty affected Fosu-Mensah, he was insecure in his movement,” van Gaal said.
“He needed to deal with the pressure and managing his position in relationship to Barkley and [Tom] Cleverley.”
Fosu-Mensah has enjoyed a superb start to his United career but he would have been disappointed to concede the penalty against Everton.
The episode will come as a setback but there is no doubting that the 18-year-old will put it behind him before one day becoming a top player.