Marcus Rashford, 18, scored the winner for United in the Manchester derby on Super Sunday, bringing them to within a point of City in the race for the top four.
Jesse Lingard also put in a fine performance, alongside Anthony Martial, and Souness gave Van Gaal credit for sticking with the youth players for such a big game.
"I think United will be much encouraged by the way the young players played in the first half. When City were going after them, it showed [United] had pace and imagination on the break. I think Martial is going to be an absolute star, given he's only 20 years old.
"United were really good today, and I think the manager deserves much credit. We get after him all of the time but he deserves credit.
"Sticking with these young guys, I think we picked holes in playing the young ones all the time before the game, but they came good for them today.
Graeme Souness "United were really good today, and I think the manager deserves much credit. We get after him all of the time but he deserves credit."
"First half they were very threatening on the break, obviously they expected to spend a lot of time without the ball.
"In the second half, after getting their goal, City have huffed and puffed but United stood firm.
"When you think De Gea had only three saves to make on 90 minutes, tells you the guys in front of him did a really good job."