The Spaniard found thebackofthenet three times in the space of 47 minutes in a dominant display from Arsene Wenger's men. Alex Iwobi added a fourth for Arsenal before Seydou Doumbia pulled one back for Basel.
Isak wouldn't miss a second opportunity though, with the former Real Sociedad goalscorer reacting quickly to a rebound in front of goal and placing a shot into thebackofthenet.
The Egyptian carried his run and kept his composure to slot the ball into the bottom corner ofthe Chelsea net. Chelsea battled back and grabbed an equaliser through Willian whose cross lobbed Simon Mignolet.
Arsenal wasted the chance to go top ofthe Premier League as West Brom fought back from a goal down to beat them at The Hawthorns. The visitors took the lead through Olivier Giroud, who headed in Mesut Ozil's free-kick from the right.
Ahead by two goals from the first leg in Copenhagen three weeks ago, any lingering hopes of a shock were ended within nine minutes as Manuel Akanji and Julian Alvarez found thenet.
Barca came back with urgency after the break and eventually equalise thanks to a mistake by Rayo defender Lejeune, who sent the ball into his own net with the tip of his boot as he tried to clear a cross in the 82nd minute.
After back-to-back 3-0 defeats against Manchester City and Newcastle, questions were increasingly being asked ofthe manager, and eyebrows were raised before kick-off when Marcus Rashford was left out ofthe squad.
First, Diaz sprayed a ball wide to Joe Gomez before running around thebackofthe defence into the area to meet theright-back's cross and steer his header into thenet.
Spurs applied pressure as they looked to get back into the game, with striker Harry Kane striking the foot ofthe post and Ben Davies forcing a good save from Thibaut Courtois.
Bu Deportivo fought back just after the hour mark when Joselu blasted into the top corner from the edge ofthe box. Two minutes later, Depor were 2-1 up.
The England international made the most of an under-hit back pass from Marouane Fellaini, rounding David de Gea and rolling the ball into an unguarded net.