Mata had been Chelsea's player of the season in the two seasons leading up to Mourinho's return, but was quickly shoved to the outer fringes of the squad. Six months later and he would find himself in a helicopter on his way to Manchester United's training ground in Carrington.
Mourinho sold him for a cool £37million and used the money to buy Nemanja Matic and Kurt Zouma. The Serbian is now a key player for Chelsea while Zouma is growing into an impressive central-defender.
The young Frenchman has managed to fill a gap left by Luiz, who was sold to Paris Saint-Germain in the summer transfer window for a reported fee of £50m. Both Luiz and Mata were considered key players when they left, but Chelsea have grown stronger without them.
"He also knows when players take their foot off the gas"
And now Terry has given his back to Mourinho over his transfer policy, claiming that the Special One seems to sense exactly when a player needs to be sold.
He told The Sun: "You look at some of the world-class players we’ve had, Juan Mata, David Luiz, players like that who’ve left the club."
"He’s seen something — I don’t know what — to let those big players go. He also knows when players take their foot off the gas."
Chelsea fans would be mad to suggest that Mourinho was wrong to accept PSG's huge bid for Luiz, but his decision to sell Mata to Manchester United seemed a risky one. The playmaker had been superb prior to Mourinho's arrival and was likely to do the same for their Premier League rivals.
However, Mata has once again struggled to establish himself under the stewardship of a new manager. Louis van Gaal has used the Spaniard sparingly this season with Wayne Rooney being prefered in Mata's favoured No.10 role.
But he is threatening to break through after a match-winning brace against Liverpool last weekend. The diminutive playmaker could yet come back to haunt Chelsea and Mourinho in the future.
Would Chelsea fans have liked to have seen Juan Mata stay or was Jose Mourinho right to cash in on him when he did? Give us your views in the comment box below!