The Atletico Madrid boss watched most of the game from the stands.
Diego Simeone, if reports are to be believed, is one of the names in the frame to replace Arsene Wenger as Arsenal manager at the end of the season.
With his Atletico Madrid side facing the the north London club in the Europa League semi-final on Thursday evening, he was given a taste of what it was like to take up a position on the sidelines at The Emirates. Unfortunately for him, it didn't last too long.
Already furious at the dismissal of Šime Vrsaljko after just nine minutes, the former Argentina international reacted angrily when an Arsenal player's mistimed challenge didn't receive a yellow card moments later.
Cameras caught the majority of his rant at the referee, Clement Turpin of France, which swiftly saw him sent to the stands.
But what, exactly, did he say? Well, although we're in no position to say, we can at least point you in the direction of social media, which always seems to have a couple of willing lip readers in these kind of situations.
A number of viewers seem to think Simeone shouted the term "Hijo de puta" at the official, loosely translated to "son of a bitch".
https://twitter.com/Matt5cott/status/989584712216834048?ref_src=twsrc%5Etfw&ref_url=https%3A%2F%2Fwww.givemesport.com%2F1301856-fans-think-they-know-what-diego-simeone-said-after-being-sent-off-against-arsenal&tfw_creator=GiveMeSport&tfw_site=GiveMeSport
https://twitter.com/Kalvapalle/status/989585000327729152
https://twitter.com/hectorirl/status/989586873101234181
It's not clear at this stage if this is accurate or if the referee even understood what he'd supposedly said. It's entirely possible he was ordered away purely because of the aggression he showed.
