The former Liverpool man believes his teammate is happy with life at Camp Nou and will resist a return to the Premier League in favor of remaining with the Liga club.
United has already has two bids rejected for the former Arsenal man as new boss David Moyes sets about strengthening his side's midfield.
However, Mascherano believes Fabregas is happy with life back in Spain and will be a key player for the club under new coach Gerardo Martino.
"I didn't talk to him but I think he is happy," he 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.”