Referee Anthony Taylor shocked the Chelsea players and fans when he overruled the linesman’s decision to let Alexis Sanchez’s goal stand in the 4th minute.
Aaron Ramsey was in an offside position when Sanchez scored, but the referee judged that he didn’t interfere with play.
See the offside rule from the IFAB Laws of the Games for the 2016-17.
Law 11: Offside
A player in an offside position at the moment the ball is played or touched by a team-mate is only penalised on becoming involved in active play by:
- interfering with play by playing or touching a ball passed or touched by a team-mate or
- interfering with an opponent by:
- 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 or
- clearly attempting to play a ball which is close to him when this action impacts on an opponent or
- making an obvious action which clearly impacts on the ability of an opponent to play the ball
- gaining an advantage by playing the ball or interfering with an opponent when it has:
- rebounded or been deflected off the goalpost, crossbar or an opponent
- been deliberately saved by any opponent
- A player in an offside position receiving the ball from an opponent who deliberately plays the ball (except from a deliberate save by any opponent) is not considered to have gained an advantage.
Ramsey didn’t touch the ball but interfered with play and caused confusion for Courtois as he went for the ball at first. He made an attempt for the ball which affected Courtois’ judgement.