The Europa League last-16 has thrown up a mouth-watering first continental clash between the north-west rivals, but the fact they are going toe-to-toe in this competition only underlines how far the most successful clubs in English football history have fallen recently.
United and Liverpool have won 20 and 18 league titles, respectively, along with a combined eight European Cups - a history of success that Van Gaal believes came during a bygone era.
"In football, it's normal," the Reds manager said of teams falling from the summit.
"It's not normal that one team is dominating for 20 years in a row (as) the champion."
When put to Van Gaal that Liverpool and United had dominated for decades, he responded: "It was another time, it was another time.
"You live in the past, you live in the past and you have to live in the present, I think."
That current state of affairs domestically means the Europa League may well offer both clubs their best chance of reaching the Champions League.
United sit sixth in the Barclays Premier League, three points off the top four, and Liverpool are a place and three points worse off, albeit with a match in hand.
However, Europa League glory would guarantee a place at European football's top table regardless of league performance, adding extra motivation - if any was needed - for the last-16 tie.
"Liverpool - Manchester United is always a big game traditionally," Van Gaal said ahead of Thursday's first leg.
"It's historic and, especially because both teams fight for a Champions League position, it's a big game for both sides."
Thursday's trip to Anfield offers Van Gaal the chance to win a fifth successive competitive match against Liverpool.
It is feat that would fill the Dutchman with pride, even if he has seemingly divorced himself from the magnitude of emotions attached to the derby.
"I think football in England is very deep in the life of the people," the United boss said.
"Of course I have noticed that also in Spain and in Germany, but that was less so deep than here so it is fantastic to see that.
"That's also one aspect as to why I wanted to come here, to have that experience.
"Sometimes I can see too much but I like it to see (it) because you can see the heart of the club and also the feeling that they are with you every game.
"So that is nice for the players, I think, but also nice for the manager.
"Not all the people are like that but several people are thinking they are the enemy.
"I am not thinking like that, I am thinking like they are an opponent and a lot of fans want to beat the opponent."
Source: PA