The England international, who was slammed earlier on in the season for diving, appeared to go to ground easily in the box during United's goalless draw with Real Sociedad in Spain.
"I was disappointed with Ashley Young," McGrath told talkSPORT. "That was never a penalty. I genuinely believe he is a very good player but he's got into this habit of tripping himself up if anyone bumps into him in the penalty area.
"It takes away from the fact he's a brilliant footballer. He's going to be left with the perception he's the one who always dives in the penalty area."