Rafa Benitez wasn't the most popular Chelsea manager in the club's history - despite winning a Europa League title.
So you can't really blame him for feeling a little bitter, even when he's clearly desperate to avoid any controversy.
As reported by the Daily Mail, when asked about Chelsea's January transfer business at a Napoli pre-match press conference, Benitez clearly tried to swerve the question.
But he couldn't help himself, and took a little shot at Chelsea boss Jose Mourinho for old times sake.
"I am not here to talk about Chelsea," said Benitez. "But Juan was their best player in the last two years. That is it. You can see that, but I am not here to analyse what they did."
Mata was twice named Chelsea player of the year at Stamford Bridge, but Mourinho allowed him to leave for Manchester United in a £37m deal last month.
Following the arrival of Mohamed Salah, Mata was deemed surplus to requirements - and Mourinho made a point of Chelsea's new commitment to balancing the books when it comes to transfers.
Benitez was a big fan of Mata during his time at the club, and the Spaniard was a key player as Chelsea clinched a top four spot and a Europa League victory over Benfica in the final last year.
Mata, now at United, is still finding his feet at Old Trafford. He arrives at the club at a difficult point, with the defending Premier League champions struggling in the league.