Van Gaal signed Robben, 30, from Real Madrid when he was Bayern manager in 2009 and also worked with him during his time in charge of the Netherlands squad.
And now the Dutchman is keen on a reunion at Old Trafford and prepared to spend £24m to do so - but the Bundesliga giants won't let him leave unless he requests a transfer.
United have received a boost in their pursuit of Real star Gareth Bale, who is reportedly set for a big money move next summer.
Spanish journalist Manuel Esteban believes they're looking to offload the Welsh winger to raise funds for Borussia Dortmund's Marco Reus and Manchester City's Sergio Aguero.
GETTY
Louis van Gaal wants Manchester United reunion with Arjen Robben
Do we have to buy (defensive) players? No
Louis van Gaal
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He said: "Gareth Bale will not continue at Real Madrid. If they receive a big bid, they will not be hesitant."
Van Gaal has revealed he has no intention of strengthening his defence when the transfer window reopens in January, despite being heavily linked with moves for Diego Godin, Mats Hummels and Ron Vlaar.
He said: "When you have injuries you cannot solve the problem otherwise," said Van Gaal.
"Do we have to buy (defensive) players? No. We have more than enough."