Rooney's 79th-minute strike will be a contender for goal of the month, if not the season.
The England captain brought down substitute Angel di Maria's cross with his left foot and then turned sharply before sweeping the ball past Brad Guzan on the volley with his right.
"It was beautiful - and also the assist of Di Maria," Van Gaal said. "He was not on the pitch long and he already has an assist. That's fantastic
"It was his 10th assist this season today and he hasn't played all the game like Herrera did so it's a fantastic result.
"Like Di Maria his time is coming but he has to wait for the right moment."