But he did not rule out resting him at some stage "when we think it is better".
The England striker has looked below par in recent games and drawn criticism from many supporters who feel he should either be dropped or moved into midfield, allowing French prodigy Anthony Martial to take over as the main attacker in United's 4-2-3-1 formation.
On Monday, however, Van Gaal defended his "fantastic" captain, who has scored only two goals in the Premier League this season, telling a news conference that Rooney has "more credits" than any other player.
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United have drawn their last four games, the past three of them goalless, and won only one of their last six.
They have scored only 15 goals in 11 Premier League games and Martial's header in the 1-1 draw at CSKA two weeks ago was the last time they found the net.
"We've had a dip in our results and we have to come out of it," Van Gaal said. "That's why we need the support of the fans.
"I hope they shall support my players. I hope they will be the 12th player again tomorrow.
"Against Crystal Palace (on Saturday) was a concern because we don’t create so much but that was one of a few matches that we were not the better team.
"We are the best defenders of the Premier League, you say we cannot attack and it’s not true. It’s only a moment in a process.
"It’s a bad period when you don’t score. Football is scoring goals."
United have a win, a draw and a defeat from their Group B matches so far, the same as Tuesday's opponents.
Midfielder Morgan Schneiderlin did not train on Monday for "personal reasons" the manager said, adding that defender Phil Jones was fit to play but not Memphis Depay.
(Reporting by Steve Tongue; Editing by Mark Meadows)
