Messi led Argentina to his first and his country's third World Cup triumph in December 2022, as the star forward scored twice in the final to help the South American nation beat France 4-2 on penalties at the Lusail Stadium.
Argentina had earlier knocked Netherlands out of the competition at the quarterfinal stage after beating them on penalties following a 2-2 draw.
Van Gaal, who resigned as the Dutch head coach following their World Cup exit, said he thought their game against Argentina was premeditated.
"I do not really want to say much about it," Van Gaal told reporters. "When you see how Argentina scored their goals and how we scored our goals, and how some Argentina players overstepped the mark and were not punished, then I think it was all a premeditated game."
Asked to clarify what he meant, Van Gaal said: "I mean everything I say."
When asked again if he meant that Messi should become world champion, Van Gaal told the reporter: "I think so, yes."
There was tension between Messi and Van Gaal during the match after the forward celebrated his goal by running in front of Netherlands coach and cupping his hands to his ears.
The Argentina captain also approached Van Gaal after the penalty shootout and Messi later said he regretted his actions in the game.
Dutch captain Virgil van Dijk, who played against Argentina at the World Cup, disagreed with his former manager's comments.
"I heard it this morning, indeed. And that's actually it," he told NOS. "It is of course his opinion. Everyone is allowed to have an opinion. I do not share the same opinion."
Information from Associated Press was included in this report.