Arsenal kept their slim hopes of a top-four finish alive with a comfortable 2-0 win over a much-changed Manchester United side at the Emirates Stadium on Sunday in a first league win for Arsene Wenger over Jose Mourinho.
The game began at pace and United -- who suffered their first league defeat since October -- were first to show when, only two minutes in, Juan Mata crossed and Wayne Rooney drifted away from his marker but steered a header over.
Within three minutes, Kieran Gibbs got down the left for the hosts and Welbeck's attempt to control saw the ball fall to Sanchez, who got past a defender and had his effort stopped by David De Gea.
Back came United, and Rooney slid a ball through to Anthony Martial, whose strike was turned behind by Petr Cech -- and before 10 minutes had been played there was another opening as Aaron Ramsey shot low and De Gea made an outstanding save.
As the quarter of an hour mark approached, Danny Welbeck almost got through for Arsenal but was denied by Ander Herrera after he had drifted away from Phil Jones and cut into the area.
But United were beginning to enjoy more of the possession and, with 21 minutes gone, a deep Mata free kick to the far post was headed on by Henrikh Mkhitaryan only to elude his teammates.
Mata was involved in another opening, getting away down the right but crossing too close to Cech as Mkhitaryan waited unmarked in the middle.
Then Welbeck had an effort on the turn blocked by Jones six yards out after chaos from an Arsenal corner, and on the half-hour Chris Smalling made a crucial intervention at the near post after Ramsey had got away and crossed low.
Arsenal kept pressing, and after some fine footwork by Alex Oxlade-Chamberlain had bought him space on the edge of the area his shot drew another good stop from De Gea.
But United should have led on 32 minutes when Rob Holding's dangerous pass back into his own area was seized upon by Rooney as Laurent Koscielny hesitated, only for Cech to block.
The second half started in sleepy fashion, with youngster Axel Tuanzebe called into its first piece of action as he fended off a raid from Sanchez down the Arsenal left.
But after 54 minutes the home side broke through when Granit Xhaka picked up possession 30 yards out and his shot took a big deflection off Herrera and looped over De Gea.
And three minutes later the lead was doubled as Welbeck struck against his old club, heading home a pinpoint Oxlade-Chamberlain cross.
Mourinho reacted to the double strike with a double change, taking off Mkhitaryan and replacing him with Jesse Lingard and, moments later, introducing Marcus Rashford for Herrera.
Rooney forced Cech into action with a curling free kick from 25 yards after 65 minutes, but United were doing little to worry Arsenal as Wenger's side sat comfortably on their lead.
Ramsey won the ball back deep in United territory as the Gunners looked to add a third, but when he laid it back to Sanchez the cross was blocked.
Ozil crossed from the left but just ahead of the waiting Welbeck before Rooney shot wide as United laboured in vain for a way back into the match.
Xhaka was replaced by Francis Coquelin for Arsenal and Rooney shot over again when he had both time and space on the edge of the box.
Oxlade-Chamberlain was replaced by Hector Bellerin and Olivier Giroud came on for Welbeck as the home side made a pair of changes, while Scott McTominay made his senior United debut when he came on for Mata.