United's record goalscorer Rooney was captain under Van Gaal at Old Trafford and scored 29 goals across all competitions in the Dutchman's two seasons in charge.
However, Van Gaal felt he did not see the best of Rooney during his time at United and seemingly questioned his approach away from football.
Speaking to the Guardian, Van Gaal said: "I'm sorry but he was over the hill. But in spite of that he was one of my best players.
"You can say nothing about his professionalism as a player in the training sessions or on a pitch.
"Outside the environment of the training pitch and the stadium is different. I made him my captain because I want to control that life outside.
"I think we don't succeed totally."
Van Gaal feels United's post-Alex Ferguson struggles are mainly due to the club being unable to freshen up the first-team squad.
The former Barcelona boss signed the likes of Angel Di Maria, Bastian Schweinsteiger, Anthony Martial and Memphis Depay during his tenure but hinted the club's hierarchy did not necessarily land the targets he wanted.
"The problem begins with, of course, that Manchester United was never refreshed. I think when you are a manager you have to refresh every year to keep the team-building process going," he said. "I didn't always get the players that I want. That's the problem. There is [executive vice-chairman Ed] Woodward and his right hand is [head of corporate development] Matt Judge.
"Judge I met once in a while but not too much. And there was the head of scouting. That was the structure, but you are always dependent on Woodward and Judge.
"I thought always Manchester United can buy every player because they have a lot of power.
"Seemingly a few players were not reachable for Manchester United. I cannot understand but it was like that."
On Di Maria specifically, Van Gaal felt the Argentina international – who lasted just one season in the Premier League before joining Paris Saint-Germain – was not suited to English football.
"I was satisfied, because he was a creative player, but I had other players on the list. Di Maria had a problem with the English football culture and the climate," he said.
"You cannot buy players and know, for sure, that they can deliver. You cannot know because football is a team sport.
"I always ask a player where he wants to play. For him it was wing, wide and mostly left. In the Argentina team he plays on the left. I started with him there.
"He was not performing that well, to a level you could expect from an 80million-pound player. I believe, then, I have to see if another position is better for him.
"I have played him left winger, as the 10, second striker and on the right. Then the critics say he is having to play in too many positions. I gave him all the chances that there were to perform well."