After taking his place on the bench, Sanchez watched on as Arsenal conceded a fourth goal and the forward was spotted smirking, which drew the ire of Sky Sports commentator Gary Neville.
"I doubt he's aware the camera's on him. Not sure what he's smiling about," said Neville, before adding, as the camera panned towards disappointed Arsenal fans: "That's more representative of what he should be looking like."
Sanchez, who was making his first appearance of the season, played over an hour in the 4-0 loss before being replaced by Olivier Giroud. However, his demeanour upon his return will have done little to win over fans.
Interestingly, the former Barcelona man was guilty of a similar indiscretion against Liverpool last season, which led many supporters to the conclusion that he was on his way out of the Emirates Stadium.
Sanchez is on the radar of a number of clubs, including Manchester City, and Arsenal boss Arsene Wenger appears to be resigned to losing the 28-year-old on a free transfer next season as he refuses to sign a new deal.