Messi, who has become a target of supporters' frustrations following a perceived lack of effort, was reportedly jeered by certain sections of the Camp Nou during the Catalan club's final league game of the season last weekend.
"Leo doesn't seem down at all," the Argetine's fellow countryman Mascherano told TV network TyC Sports. "I'm lucky enough to spend every day with him and I think that on a personal level he's in great form.
"He seems fine to me. Sometimes things go right and other times they don't. This is a team sport and I don't think Barcelona have given him what he needs this year, but the squad in general have not played as well as in previous seasons. It's as simple as that.
"Leo is the best player in the world, but he's just one player and he needs a team behind him that allows him to shine and I don't think this has been our best season. We have to be realistic."
Messi recently agreed to sign a new contract with the Spanish club.