Manchester United lost 1-0 to Juventus on Tuesday night thanks to a Paulo Dybala strike.
It was the Red Devils’ first defeat in three games and casts worry over their hopes of reaching the Champions League knockout stage.
Mourinho has caused a storm off the field in recent weeks, while he is the subject of goading from rival fans on the touchline.
And he reacted to Juventus fans shouting “Mourinho s****y man” and “Mourinho son of a b****” last night.
The Portuguese boss heard the chants and raised his three fingers in the direction of the away fans.
It is in reference to the treble – the Serie A title, Coppa Italia and Champions League – Jose Mourinho won while Inter Milan manager in 2010.
Asked about the salute after the game, Mourinho said: “It’s an obvious response.
“Obviously, they don’t love me. Obviously, the toughest moment for them was our treble (in 2010).
“That’s fine. With players, coaches, club and everything, we have total respect, everything is very cordial.”
Mourinho’s three-finger salute has already been seen this season.
The manager brandished his newly-acquired retort at Chelsea fans during the 2-2 draw at Stamford Bridge on Saturday.
That was in reference to the three Premier League titles Mourinho won with Chelsea during his two stints in charge of the club.
Mourinho may enjoy reflecting on past glories but his United side need to start getting results suck if he is to remain in the job much longer.
The manager has overseen just one win in United’s last seven games – and that was a dramatic late comeback from two goals down against rock-bottom Newcastle.
Reports in Spain claim Mourinho is wanted at Real Madrid, who are set to sack manager Julen Lopetegui.
Lopetegui’s Real beat Plzen 2-1 last night but pressure is mounting on the former Spain boss, who may be axed before Sunday’s Clasico with Barcelona.
“This question is not for me,” Lopetegui said when asked if he thinks he will be sacked.
“We are thinking about preparing for Sunday’s game which is very important, and we will go there with great ambition.”
The Portuguese boss managed Real between 2010 and 2013, winning La Liga once during that spell.
Next up for the Red Devils is a home tie with Everton on Sunday, before a trip to Bournemouth a week later.
They then travel to Juventus and Manchester City before the next international break.