The former Camp Nou manager believes the Argentina star and Neymar should aspire to help their team-mates like the Liverpool midfielder.
Lionel Messi and Neymar limit the success of Barcelona and Paris Saint-Germain because they are not "team players", according to former Blaugrana coach Louis van Gaal.
Messi is considered one of the greatest players of all time, while Neymar is the most expensive signing in history, yet Barca have not won the Champions League since 2015. PSG, meanwhile, are still waiting on their first such title.
And Van Gaal, who coached at Camp Nou from 1997 to 2000 and 2002 to 2003, believes the two individuals must take responsibility for these failings.
"Look at Barcelona. How many Champions Leagues have they won with what they say is the best player in the world?" he told El Pais.
"Look at Neymar at PSG. How many Champions Leagues have they won? I like Neymar and Messi as individual players but not as team players. I believe that in team games, there is nothing more important than the team player."
Extending his point, Van Gaal compared Messi and Neymar unfavourably to versatile Liverpool midfielder James Milner.
"One of the best is James Milner," he said. "In the final of the Champions League, he played as a defender and as a midfielder. It's great that he can offer that at 33.
"I think Messi should ask himself how it is possible that he has not won a Champions League for so long.
"I am neither in the dressing room nor in training at Barcelona. I cannot judge. I like Messi as an individual player. He is the best individual player in the world because his statistics are amazing. I like him.
"But why has he not won the Champions League since five years ago? Why? As a captain, you must ask yourself why the team does not win in Europe.
"Barça have a wonderful squad. You cannot say that [Ivan] Rakitic is bad, or that [Philippe] Coutinho is bad, or that [Jordi] Alba is a bad player, or [Marc-Andre] Ter Stegen a bad goalie, or Arthur or [Arturo] Vidal.
"I think Messi is also responsible for what is happening in Barcelona - not only the coach. Players have a substantial part of responsibility for what happens on the team."
Van Gaal: "Neymar played for Messi's service at Barça"
Van Gaal was not overly concerned by reports Neymar isset to rejoin Messi at Barça, yet he does feel the Blaugrana captain is given too much freedom by his coaches.
"I remember that Neymar at Barca played for Messi's service," the ex-Manchester United boss said. "I'm not against Neymar. He is fabulous. But at PSG, he does not play for the team.
"And I believe that every player must play for the team - even Messi. [Barcelona] have a staff of 30 players and I think Messi must adapt to the team and not the other way around.
"[Pep] Guardiola did play for the benefit of the team but recent coaches have adapted too much to Messi instead of protecting the team spirit. That is the most important."
Assessing Barca's 4-0 semi-final second-leg loss to Liverpool, Van Gaal said: "If there is a true team spirit, at the highest level this result is impossible."
On PSG, he added: "They do not have the right team spirit. It is obvious. I saw them at the Parc des Princes the day they beat Bayern 3-0 in the Champions League in 2017.
"I saw them on the field: the team-mates were not happy with Neymar's style of play. And they are still not happy."