The Belgian midfielder's backpass dropped far too short for David De Gea, allowing Jamie Vardy to nick in and score Leicester's equaliser on 52 minutes after Jesse Lingard had netted a stunning individual opener.
After the match, which was won by Zlatan Ibrahimovic's 83rd-minute header, Mourinho spoke of the Fellaini pass which he felt was negatively affected by the slowness of the grass at the FA's headquarters.
"I think it's the mistake of the slow pitch," he said on BT Sport and MUTV.
"Before the game starts I put my hand on the surface and I thought, 'Wow, we are in trouble because we want a fast pitch and the pitch is really slow.'
"Good pitch but slow. I don't know if they put water on and the atmosphere killed the water immediately or if they forgot to do it but the reality is it was very slow.
"So Marouane gives a backpass and the ball slowed down and Vardy smells everything and is really fast."