The Italian defender scored an impressive left-footed volley to open his scoring account for United on Wednesday night.
However, Van Gaal has claimed that Darmian showed impressive shooting skills in United training sessions this week, hinting that a goal was only around the corner.
"Darmian was surprising me on Monday in the training session," Van Gaal said.
"We did a sort of finishing session and he was the top scorer in that time and we tried to exercise with his left and right foot and he did it also with his left foot like he did tonight.
"I am very happy."