Van Gaal faces the toughest test of his reign as Manchester United manager on Sunday when unbeaten league leaders Chelsea come to Old Trafford.
In the away dugout will be a man Van Gaal first crossed paths with 17 years ago when the Dutchman offered the then 34-year-old coach a position on his coaching staff at Barcelona.
Mourinho spent three years by Van Gaal's side and the relationship worked perfectly, with Barca winning two Primera Division titles and the Copa del Rey while the two men were stationed in the home dugout.
Mourinho has since forged a hugely successful career of his own which has yielded an astounding 20 trophies to date.
The Portuguese often credits a lot of his success to Van Gaal and his other mentor Sir Bobby Robson, but the Manchester United manager thinks the main reason behind Mourinho's success is the man himself.
"He talks about Louis van Gaal and also Bobby Robson, and I appreciate that, but you always do it by yourself," Van Gaal said.
"I was also an apprentice when I was the assistant coach of Leo Beenhakker (at Ajax) and it's good that my eyes were also open at that time.
"And it's good that Jose Mourinho's eyes were very open when he was my assistant, but you do it by yourself."
Mourinho has riled many leading figures in football, whether it be in Portugal, Italy, Spain or England.
Just three weeks ago he was tussling with Arsene Wenger, the man he called a "specialist in failure" in February.
The 51-year-old has been viewed as a somewhat arrogant figure ever since he proclaimed himself the 'Special One' when he arrived at Chelsea 10 years ago, so it was with some surprise that Van Gaal chose to describe Mourinho as "modest" at his pre-match press conference on Friday.
And, anyone who turned up to the media briefing hoping to hear about a breakdown in relationship between the two men due to the fact that they are now domestic rivals, was disappointed.
Indeed, Van Gaal had nothing but kind words to say about his former assistant.
"He's such a wonderful coach," Van Gaal added.
"And to see that he's been a champion in different countries is fantastic.
"He is a very modest and emotional human being, and I like that.
"He's very special but he is very special for me because I've worked with him and we've continued our relationship, which in the football world is not always normal
That's nice."
One of the hallmarks of Mourinho's management career has been his propensity to engage in mind-games with his rivals.
The Portuguese has kept the public guessing with his updates on the fitness of star striker Diego Costa in recent weeks.
Mourinho said he did not expect Costa to start against United when he spoke following Chelsea's win over Maribor on Wednesday, but at his pre-match press conference two days later, he said the striker had a "little chance" of recovering from his illness to play.
Van Gaal appears to believe the Spain striker, who moved to Stamford Bridge in a £32million move from Atletico Madrid this summer, will start.
"I think that he [Mourinho] shall play with him when he can because he wants to win," the Dutchman added.
"Of course he can play maybe because he has trained individually, I have heard and read (that) on the websites
I also have my eyes always open!
"He's scored nine out of seven - that is remarkable
So he's in good shape, I think, but he was ill yesterday, so you never know."
Van Gaal confirmed United's own star Angel di Maria, has been passed fit after he overcame the thigh injury he suffered in the 2-2 draw against West Brom on Monday.
The draw at the Hawthorns underlined the difference between United and Chelsea so far this term.
Thanks to the addition of Costa and Cesc Fabregas, the Blues have brutally swept almost everyone before them so far this term and rightly sit atop the Barclays Premier League.
United, on the other hand, have performed inconsistently, leaving them some 10 points behind the league leaders, much to Van Gaal's disappointment.
"Chelsea are one of the top teams," he said.
"They are playing like a team and I want my team to do that as well, for a full 90 minutes."
Source : PA
Source: PA