A player in an offside position is only penalised if, at the moment the ball
touches or is played by one of his team, he is, in the opinion of the referee,
involved in active play by:
• interfering with play or
• interfering with an opponent or
• gaining an advantage by being in that position
To me, Sakho is interfering with an opponent (Rose) AND gaining an advantage (trying to score). Offside.
However, further through the LOTG theses things are defined:
Definitions
In the context of Law 11 – Offside, the following definitions apply:
• “interfering with an opponent” means preventing an opponent from playing or being able to play the ball by clearly obstructing the opponent’s
line of vision or challenging an opponent for the ball.
• “gaining an advantage by being in that position” means playing a ball
i. that rebounds or is deflected to him off the goalpost, crossbar or an opponent having been in an offside position
ii. that rebounds, is deflected or is played to him from a deliberate save by an opponent having been in an offside position.
So Valencia (not Sakho - he scores the goal) is definitely not gaining an advantage according to the LOTG - he has to play the ball for that to happen.
It would be difficult to suggest that Valencia is preventing Rose from playing the ball as Rose doesn't even try to do so - he just sticks his hand in the air. I think if Rose had jumped with Sakho rather than standing and admiring him he would probably have got the offside decision as Sakho's challenge from an off-side position could have been seen to prevent Rose from playing the ball.
Fair goal. Poor defending by Rose.
.