Casemiro believes Neymar "doesn't have to prove anything to anyone" and wants him at the World Cup, if the striker is fit.
Neymar is Brazil's all-time top scorer with 79 goals and has played in three World Cups for the Selecao, but he has not played for them since October 2023.
He suffered an anterior cruciate ligament (ACL) injury while representing Brazil and returned to action a year later for Al-Hilal before leaving and going back to his boyhood club, Santos.
However, despite making 39 appearances and scoring 15 goals since his return to Brazil, he has not been selected for the national team.
Brazil head coach Carlo Ancelotti, who will be announcing his squad later this month, has insisted a number of times that Neymar's World Cup participation will be dependent on whether he is fully fit.
And Casemiro claimed Neymar “has to go” to the tournament if he shakes off his fitness issues, especially given the quality he can bring to the team.
"That topic is annoying," Casemiro told ESPN when asked about the Neymar situation.
"It's annoying because everyone talks about it, everyone asks about it, and everyone answers, but it's really annoying. Mostly me. I am sincerely a friend of Neymar.
"I'm a guy who has played with Neymar since I was 12-years-old. And so, it goes like this: Is Neymar going or not? And everyone fixates on that.
CONTAGEM REGRESSIVA!
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Já pode riscar o calendário porque tem convocação para Copa do Mundo e Brasil em campo!
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"But then, I think it's very clear, especially in my opinion, it's very clear. Neymar doesn't have to prove anything to anyone.
"And Ancelotti and everyone else made it very clear, it's the physical issue, the physical part, whether he will be able to get closest to his physical part, because in terms of talent, we don't even have to talk here about the great player he is, the great player he was and the talent he has with the ball at his feet.
"And Neymar has always demonstrated that. Neymar never played that role of wanting to be tough; no, Neymar was always a guy who wanted to have fun on the field.
"And I think that if you talk to him, in my opinion, about his desire to play a World Cup, and you propose, if Ancelotti proposes that [to be on the bench] to him, I think he will want to go to the national team, because he wants to.
"I think this [problem] is for Ancelotti to handle, and there is no better coach in the world, with more experience in the world, to deal with a situation like this.
"So, I'm sure whatever decision he makes will always be in the group's best interest."
Brazil’s World Cup campaign begins on June 13 against Morocco, with Haiti and Scotland also drawn alongside them in Group C.