Before the game Leeds fans were all talk about how they would beat United again like they fluked a win in the 2010 FA Cup.
But by half time the game was over as the kids ran riot and the veterans did the scoring to send United into the fourth round.
Grayson told the Evening Echo: “People don’t see how hard Manchester United work and how powerful they are.
“They are playing at the top level because they have that fantastic desire to do it every week.
“The team they put out could have beaten a host of Premier League sides. They caused us a lot of problems and it shows the gulf that is between us.”