"It would be terrible because of what Ney means to Barcelona," Messi told Argentina's TyC Sports on Tuesday, reports Efe news agency.
"Despite the way in which he left ... He won important titles here. It would be a hard blow for everyone, especially for all Barcelona supporters. He knows it. I've told him, we've talked about it."
Neymar, one of the attacking mainstays of the current Brazil squad, plays for French powerhouse Paris Saint-Germain (PSG).
Last August, PSG paid Barcelona a record transfer fee of 222 million euros ($263 million) for Neymar, who reportedly started talking about leaving the French club after they were eliminated from the Champions League by Real Madrid.
But the 26-year-old striker has sought to play down the talk of another move.
Turning to the 2018 World Cup, Messi said that Argentina would see the tournament as a failure if they didn't reach the semi-finals.
"We have no obligations to anyone. We're the first to want to become champions, to win. The fact that we've been in three finals and failed to become champions in all of them is a heavy burden we bear. We want to overcome that barrier," he said.
The Messi-led Argentina side lost the 2014 World Cup final to Germany and came up short in two Copa America finals.
Messi said that Brazil is most likely to win the 2018 World Cup in Russia, because they have really good players, a very steadfast defense and strikers who "kill you."
"They have strong players. They really know what they're doing. All their team moves are very mechanised," he said.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
