Rooney scored his last goal for United in the 3-1 victory over Aston Villa back in April, but Van Gaal claimed that striker had the mentality to find his scoring touch again.
The skipper did so in spades, netting three times in the 4-0 win in Brugge, to help send the Red Devils through to the group stage proper of Europe's premier club competition with a 7-1 aggregate triumph.
"I have given him always the confidence that a player needs," the United manager said.
"But I think Wayne has a very strong mentality and with his level he always shall come back. He proved that this evening.
"Still it's a long way for him and also for Manchester United it's not so easy, but of course I'm very happy for him."