Yamal went into the clash claiming Real Madrid "are always stealing" before posting a message to Los Merengues fans insisting they do not scare him.
But he produced a low-key performance on the night and was taunted and targeted by Real players and fans at the final whistle.
Sorg, who was in the dugout for suspended head coach Hansi Flick, said afterwards: “It could be that the whistles affected him. Maybe a little. Because he's learning to deal with the crowd, the whistling... It's a process.
"It's true that he's normally very motivated, and today wasn't very easy for him.
"I think he's coming off an injury. He needs rhythm, matches... It's normal. He's 18, and we have to give him time and help him. It's normal; opponents try everything against him, and they adapt to defend well. We have to work harder and help him get the best out of him on the pitch.
"They often defended two against one and wanted to prevent him from entering the box. That's normal. He's young, he needs to improve, and we're going to help him."