Fabregas was watching his former team and soon tweeted: “Mason Mount, fantastic.”
Chelsea went on to win, with Olivier Giroud heading home in injury time as the Blues qualified for the Champions League knockout stages with two games to spare.
It is the earliest Chelsea have reached the last-16 stage in 10 years.
After the game Mount saw Fabregas’s tweet and responded, writing: “Thanks magician, been watching your assists.”
Fabregas made 350 Premier League appearances, across spells with Arsenal and Chelsea, scoring 50 goals and supplying 111 assists.
Next up for Chelsea is a top-of-the-table clash against Tottenham.
Mount told Chelsea TV: “The gaffer has made it very known how big of a game it is. All of us boys know how big of a game it is and we’re going into the game with the confidence that we want to win.
“That’s what we’ve been doing recently and we’re not going to change. But we’re ready, we’re confident and we’re ready to fight.”