The ex-Arsenal captain has been heavily linked with a move to United, with David Moyes confirming the club's interest in the player this summer.
But Barcelona's official stance is that he's not for sale, a position backed up by senior Barca players like Song and Mascherano.
Speaking to Fox Sports Asia, Mascherano said: "I didn't talk to him, but he's happy. It's difficult when you have to talk about the personal situation of other players because sometimes you don't know what they really think.
"But I could see him here training and he's happy. He grew up in Barcelona and he feels the colours of the shirt.
"It's difficult when you grow up here to leave because Barcelona is a big team and a big city, where it's really nice to live and his family is here."
The ex-Liverpool midfielder did provide a hint of encouragement for optimistic United fans when he admitted nothing was certain regarding Fabregas's future.
But he confirmed it's his understanding that the 26-year-old remains a highly valued member of the Barca set-up.
"You can never say no but at the moment I think he is a really good player for us and the coach and the people that work here want to keep him."
And Song, another of Fabregas's teammates at the Nou Camp, had backed up Mascherano's claims when he spoke before a friendly match against Polish side Legia Gdansk earlier in this preseason.
"I know him well and everything that's been talked about is just speculation," said Song.
"He loves the club and the city and I think he wants to continue with Barca."