Fred was lucky to avoid a red card at Old Trafford after appearing to push his head into the face of PSG's Leandro Paredes.
Manchester United's Brazilian midfielder came to blows with the Argentinian with the score 1-0 to PSG in the first half following an early Neymar goal.
But even after a VAR check and a few scuffles between angry players on both teams, the referee opted to award just a yellow card.
Marcus Rashford then equalised minutes later to add insult to injury for the French visitors.
And fans on Twitter watching on were split over the pivotal incident in Manchester.
Some even pointed out how Nicolas Pepe was sent off for a similar incident in Arsenal's Premier League draw at Leeds recently.
One said: "Paredes makes the first move."
Another put: "A non-PL referee using his common sense."
A third insisted: "Straight red".
A fourth said: "Should have booked Paredes too, going down like that, embarrassing."
"It was literally the exact same situation with Pepe and Alioski," another claimed.
And a sixth tweeted: "Can’t even blame VAR. Gave the referee a second look and he still made the wrong decision".
At half time, Rio Ferdinand said in the BT Sport studio: "He's a lucky boy not to be off the pitch. I think there's been a few occasions in the first half where he could have had a yellow card.
"Obviously the headbutt. He moves his head and motions towards the player. He makes a meal of it, yes, but in today's game he's lucky to be on that pitch."
Paul Scholes added: "He's very lucky. He needs to think about staying quiet. Ole might even think about taking him off."
Ole Gunnar Solskjaer opted to leave Fred on, though, as he appeared for the second half.