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 at Camp Nou.
"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."
Speaking last week, United boss David Moyes conceded he was in the dark as to whether the Premier League champions will up their offer or train their sights elsewhere.
The Scot said: "I couldn't tell you if there will be another bid. Obviously we will take stock of it and consider what we are going to do next.
"I never at any time said I knew we would get him. I just said we had made offers.
"But what we have here is a really good squad of players already. You mustn't forget about the quality that is already here at Manchester United.
"Undoubtedly we are hoping to add to it. I am quite confident we will do, certainly before the window shuts."
Former United defender David May told Sky Sports News Radio on Monday that a move to Old Trafford would be "perfect" for Fabregas.
May said: "When good players are there it's right that United should go for them. I wouldn't say that they're desperate; Barcelona are holding on to him, and rightly so because he is a good player.
"I think it's down to Cesc at the end of the day. If he feels he's not going to get in front of Xavi and (Andrés) Iniesta then the move to United would be perfect for him."
You can hear May's full interview, in which he also talks about United's pre-season campaign, here.