Young went to ground under a challenge from Markel Bergara, who was booked for his troubles, to earn Robin van Persie a penalty, which the Dutchman put against the post.
The incident drew criticism from commentators, with Ray Wilkins in particular labelling it as “pathetic”.
But although Moyes warned Young about diving earlier in the season, the United boss this time stood up for his winger.
“I’ve seen it and the boy certainly tugs him in the box,” he said. “The referee is two yards away from it and decides to give it.
“All I know is he got a penalty kick from a decision; I don’t know about his reputation, but the referee is there and he gave it.
“The referee made decisions all night. He’s the one who saw fit to award a penalty and send a player (Mourane Fellaini) off.”
Speaking in the Sky Sports studio, former United midfielder Wilkins said Young was developing a bad reputation for simulation.
“This is pathetic,” Wilkins said. “This is as bad for me as all these over-the-top tackles we’re getting at the moment because that is a conning of the referee.
“I just wish Ashley would cut this out because he’s a good player but he’s getting a bad name.”
United’s 0-0 draw with Real Sociedad leaves the Premier League side in a three-way fight with Bayer Leverkusen and Shakhtar Donetsk to qualify for the knockout stages from Group A of the tournament.