The A-League will host United for a friendly at Optus Stadium on Saturday night.
"I'm sure that there is time to admire them and consider who you played against after the game," Popovic said.
"But we don't want them doing that during the match.
"We've got players here in our group that get disappointed when Manchester United doesn't win, and they stay up and watch the games.
"I've got one of my sons (Gabriel) that does not miss a game of Manchester United. That just shows how big and special they are as a club.
"It (playing against United) is something these players haven't experienced and may not experience again. So you want to make the most of it. You don't want to get stage fright on the day."
"I really want the players to be relaxed and enjoy it and also be focused and go out there and show our football."