The only issue is whether the Scottish Scouser (by standing offside, almost in front of Donnarumma, and ducking when the ball comes at him) is :
“making an obvious action which clearly impacts on the ability of the opponent to play the ball”.
If the tosser wasn’t there (offside) in the first place in an attempt to disrupt the goalkeeper, Donnarumma would/could have been in a different position, or had a split second more to react to the header. As soon as the ball goes in, he is clearly complaining and pointing at Robertson so he feels that his position definitely impacted his ability to save it.
My message to the scousers would be to stick that in their whining criteria based box and tick it.