The media has been awash with Sadio Mane's obvious denied goal, a thing that was a delight to Gunners fans and anguish to the Reds diehards.
I will tackle two scenarios from the match and the reason the assistant referee based on to rule out Mane goal after he pounced on a rebound ball from the crossbar or woodwork.
I'm an active referee and self-proclaimed soccer analyst, thus, I write that the second assistant referee erred in flagging off Saido Mane, owing to the fact that Mane never 'Gained Advantage' from a rebound after Firmino goal-bound ball hit the woodwork, in the process returning to Mane who tapped home for Liverpool, only for the second assistant referee to rule otherwise.
It is obvious, that the assistant referee, based on the fact that Mane 'Gained Advantage' from a rebound ball while in an offside position. I tend to defer with him because Mane was onside before Firmino struck the woodwork.
Nevertheless, before I rescind the assistant referee decision, let us look at 'when is a player judged to have committed an offside offence?'
It is worth to look at the law first, the 2018 - 2019 FIFA Laws of The Game (LoTG) states that;
It is not an offence to be in an offside position and that a player is in an offside position when he is nearer to his opponent's goal line ahead of the ball and the second-last opponent.
There is no offside if the player is level with the second-last defender or last two opponents (defenders) and that a player cannot be penalized for an offside offense if he receives the ball directly from a corner kick, goal kick or a thrown in.
Consequently, a player can be caught offside by becoming involved in 'active play' at the moment the ball is played or touched by a team-mate through:
1. Interfering with play (IWP) by playing or touching a ball passed or touched by a team-mate.
2. Interfering with an opponent (IWO) by preventing the opponent from playing the ball by obstructing his line of vision or challenging an opponent for the ball or attempting to play a ball close to him or any action that will affect the opponent's ability to play the ball.
3. Gaining an advantage by playing the ball or interfering with an opponent when the ball has rebounded or deflected off the goalpost, crossbar, match official or an opponent and deliberate save by a defending team.
The match scenarios analysis:
When Liverpool Trent Alexander-Arnold lobbed the ball over his team-mates Mane and Firmino to chase, Mane was in an offside position but, it was Firmino who was onside that played the ball first.
Thus, Mane did not interfere with active play by playing or touching the ball, as Firmino was the first to play the ball. That is the first scene, and Liverpool is on the attack in the Arsenal penalty area and no offside.
In the second scene of the play, let's look at the position of Mane before his team-mate Firmino chips the ball past Arsenal goalie Bernd Leno; no doubt, Mane is behind the ball and onside. He is not ahead of the ball.
Then Firmino hits the crossbar, the ball rebound to Mane and he taps in the net for Liverpool opening goal and to the rage of away fans, the goal is ruled offside.
Question to ponder, before Firmino hit the crossbar, what was the initial position of Mane; offside or onside? It is clear he was onside.
As a result, Mane pounces on a rebound ball from an onside position. He scores the goal is disallowed by the second assistant referee; citing that Mane had 'Gained Advantage' from a rebound ball.
I conclude, that this is where the assistant referee got it wrong because Gaining Advantage is only penalised when a player is in an Offside position, not Onside.
In addition, there was no deliberate save by Bernd Leno, because the ball hit the woodwork and he never attempted to save or stop the ball from entering into his goal.