The Dutchman also had some positive updates in defence, with Jonny Evans now available for first-team duty and Phil Jones on the road to recovery.
Rooney missed the midweek win over Stoke City after taking a knock to the knee in last Saturday's defeat of Hull City but will shrug that off for the trip to St Mary's Stadium.
Di Maria is still struggling with a hamstring problem, but Van Gaal expects the Argentina star to be back in action soon.
Evans has successfully come through a couple of games with the Under-21 side as he bids to feature for the first time since suffering a foot injury at Leicester City in September.
Jones has been absent since October and will head down the same route before being considered.
Van Gaal insists Monday's clash with Southampton is his only concern this weekend, with any thoughts of facing Liverpool the following Sunday put on hold.
"Southampton have more points at this moment than Liverpool, plus I am always occupied with the first match and that is Southampton," he declared.
The south-coast club have been this season's surprise package, currently lying third, one place above United.
"Ronald Koeman is a good coach," said Van Gaal of his compatriot.
"It is surprising that a club like Southampton is in that position, but that is always down to the performance of the coach, the staff and the players."
Van Gaal was quick to dismiss media speculation that he will be looking to embark on another major spending spree in the next two transfer windows.
"I think it is disgusting always writing about numbers," he blasted. "I don't think (chief executive) Ed Woodward said anything about it, I don't think I have said anything about it.
"It is disrespectful to my players and I don't like to talk about it.
"I have to work with the selection I have and I have respect for my selection. I believe in my players."
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