The Spaniard spent three-and-a-half seasons with the Blues, making over 135 appearances and scoring 33 goals, before signing for the Reds in January 2014.
Speaking to MUTV ahead of Saturday's battle at Stamford Bridge, Mata assessed this weekend's opposition and considered how much of the Chelsea squad has remained unchanged since he left the club four years ago.
“It’s always a special game for me against Chelsea and it will be tough - they are doing good,” Juan said. “They have a new manager and they play with more or less the same players but in a certain style.