Fabregas' future at Barcelona has been the subject of constant speculation this summer with United having seen two bids rejected for the former Arsenal captain.
Barcelona insist the midfielder is not for sale at any price, and Mascherano believes Fabregas is happy to stay in Catalonia.
"I didn't talk to him but I think he is happy," Mascherano told Sky Sports News.
"It is difficult when you have to speak about the personal situation of other players because sometimes you don't know what they think.
"I see him here training and he is happy. He grew up in Barcelona and feels the call of the shirt.
"It is difficult when you grow up here to leave because Barcelona is a big team, a big city, really nice to live in and all his family is here.
"I think he is a really key player for us and I think the coach and the people who work here want to keep him."