"But I am very convinced that he will continue with his performances, and that he shall score at the right moment again.
"You cannot expect of a 20-year-old player coming to the Premier League that he scores every week.
"I didn’t expect it either. When he came he was accelerating in the start of his campaign at Manchester United. I know that and I have to explain it to everybody because he doesn’t need that pressure to score every match."
Van Gaal concedes dropping into the Europa League would be a big blow for the three-time European Champions.
"It’s very important for the club to continue in the Champions League, and also for the players," Van Gaal said.
"They want to show their qualities at the highest podium. That’s why it’s important (we qualify)."
United will be without Wayne Rooney, Marcos Rojo, Luke Shaw, Antonio Valencia, Ander Herrera, Phil Jones and Morgan Schneiderlin for Tuesday’s game.
Nick Powell, who has only played one game for United in the last three years, Sean Goss, who is yet to make his debut, and 18-year-old defender Cameron Borthwick-Jackson are in the 19-man party that has flown to Germany.
Guillermo Varela made his debut in the 0-0 draw against West Ham on Saturday but could be thrown into the starting XI on Tuesday.
The United squad could be depleted further for Saturday’s game at Bournemouth after Bastian Schweinsteiger was charged with violent conduct for an alleged elbow on Winston Reid against West Ham.
The German is facing a three-match ban if found guilty. He has until noon on Thursday to respond to the charge.