Diego Simeone's assistant Gabi, the former Atletico Madrid captain who returned to the club's coaching staff this summer, was caught on camera making an obscene gesture towards the Real Madrid bench during a heated exchange with Antonio Rüdiger, footage from DAZN showed.
As the confrontation escalated, Gabi could be heard shouting towards the away dugout: "You're going to suck my ****." Pedro Machado, José Mourinho's assistant, gestured for Rüdiger to head back down the tunnel rather than continue the argument.
That flashpoint was one thread in a wider blow-up that spilled into the tunnel at half-time, when Atlético goalkeeping coach Pablo Vercellone and Machado were both sent off for their part in a confrontation involving players and staff from both sides.
On the pitch, Jude Bellingham clashed repeatedly with Atlético's two Argentines, according to audio captured by Movistar's El Día Después programme. His running battle with Cristian "Cuti" Romero touched on last year's World Cup semi-final, in which Argentina eliminated England:
"You talk a lot with Messi, here nothing," Real Madrid midfielder Bellingham told Romero, after an earlier exchange in which he called the defender "a coward" and Romero shot back: "Crybaby, you cried there."
A separate confrontation followed a foul by Giuliano Simeone, who told Bellingham: "Your mother's ****," to which the England midfielder replied simply: "F*** you."
Gabi, 42, made 417 appearances for Atlético across two spells between 2004 and 2018 and captained the club for six seasons before returning this summer to join Simeone's staff, replacing Nelson Vivas.
The clashes were among several ill-tempered moments in a 2-1 win for Atlético that also produced the tunnel dismissals of Vercellone and Machado, and set the tone for the row over refereeing that Mourinho pursued in his press conference afterwards.