Neymar "doesn't have to prove anything to anyone" and "has to go" to the World Cup if he's fit, Casemiro said.
The Manchester United midfielder told ESPN that Neymar remains Brazil's star player and sung his praises.
Asked about the Neymar situation, Casemiro said: "That topic is annoying. 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.
"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," Casemiro said in an exclusive interview with ESPN.
And [Carlo] 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.
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"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.
"Then he gets there and he demonstrates it on the field and demonstrates it in training. But I think that if you talk with him, I think there would have to be a conversation and say: 'Look, you're not going to play so many games, but look, at a certain game, you're going to be crucial in these 20 minutes, 30 minutes.'
"Or you're going to play that game. I think it's just a conversation. So I'll say it again, it's about what Ancelotti will think. So 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."