Andy Robertson appeared to be struck in the jaw by the assistant referee at half-time of Liverpool's clash with Arsenal.
Linesman Constantine Hatzidakis was heading towards referee Paul Tierney just after he blew up for half-time, carrying the ball with him. Liverpool star Robertson approached Hatzidakis and appeared to grab the official as he walked across the pitch.
Hatzidakis reacted angrily to that though and moved his arm back in what appeared to an elbowing motion towards Robertson's face. The Scotland international was left furious, with Liverpool players surrounding the trio of match officials as they attempted to leave the Anfield pitch.
Robertson, Diogo Jota and Jordan Henderson remonstrated with Hatzidakis and Tierney as they left the pitch. The Reds left-back was then booked by Tierney, to the anger of Curtis Jones, Trent Alexander-Arnold, Henderson and Jota.
Eventually, the quartet were led away by a member of Liverpool's coaching staff. Sky co-commentator Jamie Carragher gave the first indication of what had happened, tweeting: "Linesman elbow on Robertson!"
According to Sky Sports reporter Geoff Shreeves, Robertson was furious as he left the pitch. Shreeves claimed the Scot said: "he’s just elbowed me in the throat, the linesman has elbowed me in the throat."
While Robertson appeared to be the victim of the incident, Roy Keane blasted the Liverpool star at half-time. He accused the left-back of being a "big baby".
"Do you know what? Does he grab the linesman first? I’m not sure, but Robertson is then complaining. He should be more worried about his defending. Do you know what he is, that Robertson?" Keane told Sky Sports
"I’ve watched him a number of times, he is a big baby. That’s what that guy is. Just concentrate on the game and get on with your defending. He does grab the linesman first."
Arsenal had quickly gone two goals ahead, with Gabriel Martinelli opening the scoring after just eight minutes. The winger then turned provider, sending in a wonderful cross for Gabriel Jesus to head in the Gunners' second.
But an incident involving Granit Xhaka fired up the Anfield crowd from its slumber. Mohamed Salah then pulled a goal back for the hosts, but missed a penalty in the second-half after the incident involving Robertson.
