Torreira was dribbling under pressure from Deeney, before making a pass.
As the midfielder played the ball, he felt the brunt of Deeney’s forearm in his face, causing him to hit the turf.
The incident was initially missed by referee Craig Pawson, but he stopped play soon after following a message from his assistant referee.
Replays showed that Deeney flung his right arm into Torreira after the ball had gone, and Pawson, upon listening to what his linesman had seen, showed Deeney a red card.
The bustling striker looked shocked and reasoned with Torreira, but begrudgingly walked off the pitch soon after.
As he reached the touchline, it seemed the 30-year-old would get into another altercation as he had words with Unai Emery and Arsenal’s subs bench, but he eventually walked down the tunnel.
Watching on from the gantry, co-commentator Alan Smith said: “Well he’s so much bigger than Torreira, sometimes you just try and arm off your opponent and you catch them in the face accidentally.
“But if it was a deliberate elbow that’s a different matter entirely.
“It’s a little swipe of the arm, something he did not need to do.
“Just letting his man know he’s there but it’s a silly challenge.
“Craig Pawson had a little chat with the linesman who might have helped him.”
Deeney has had a bitter relationship with Arsenal and their fans since saying the side lacked “cojones” after beating them 2-1 at Vicarage Road last season.
“I’m not going to be one to tell Mr [Arsene] Wenger about himself, but there’s a reason that they lost and it wasn’t because of one penalty,” Deeney said on BT Sport after the game.
“I have to watch what I say, but it’s [having] a bit of cojones, is what I’ll say. Whenever I play against Arsenal, I’ll go up and think ‘let me whack the first one and see who wants it.”
The striker was taunted when the Gunners avenged the result at The Emirates later in the campaign, but he revealed in Tales From The Vicarage Volume 7: Captains that he does not regret igniting the rivalry in that manner.
“Hindsight’s a wonderful thing, and I might have been better off just saving it for the boozer afterwards, but I don’t ever regret it,” he said.
“If I’m out in London now, I get Arsenal fans coming up to me and saying: ‘That cojones comment was out of order, but fair play for saying what you think.'”
Before Monday night’s fixture, Gary Neville urged Arsenal to use Deeney’s past comments as fuel for a result.
“I would use Troy Deeney’s comments as an inspiration if I was an Arsenal player,” Neville said on Monday Night Football. The idea of being told you’re soft by one of your opposition players is quite unique.
“We enjoyed it obviously because it’s very rare that you get a player being so open about an opponent when they’re thinking negative things. But he was open about it but tonight he’s going to have to go and back it up.
“I don’t think it will worry him too much Troy Deeney, I don’t think he’s the type of character that it will worry him. But from an Arsenal point of view it would really inspire you to not let him score, to not let him have a chance even.”