Mathieu also quit Barca at the end of last season and spoke of his former teammate's decision to leave for PSG.
He told Efe: "At no time did people think that Neymar would leave Barça.
"It's football, it's like that, it's his decision. My opinion is that he left because he was in the shadow of Lionel Messi, and I think he wants to win a Ballon d'Or.
"Today he is happy in Paris because he scores goals."