ARSENAL moved to third in the table after easing to victory over the relegation strugglers Swansea.
Olivier Giroud put the away side in front when he reacted quickest to Mesut Ozil's blocked header in the six yard box and fired past Lukasz Fabiański.
The Frenchman then signalled to come off with an injury but managed to continue until after half time.
Arsenal doubled their lead quickly after the break with Alex Iwobi's strike looping over Fabiański via a big deflection off Jack Cork.
Then in the 67th minute Kyle Naughton turned in Iwobi's cross-shot into his own net.
Olivier Giroud picks up an injury after scoring for Arsenal
Sat, January 14, 2017Olivier Giroud appeared to pick up an injury just after scoring for Arsenal against Swansea
Sanchez then got his 14th league goal of the season when he reacted quickest to a loose ball.
But there will be some concern for Arsene Wenger after Sanchez's reaction to being substituted despite the 4-0 scoreline.
The Chilean shook his head as he came off and barely acknowledged his manager on the touchline.
Then once on the bench he covered his face with his coat and could not disguise his unhappiness.