Arsenal dominated possession but was stunned just before the break when Stanislav Dragun got his head to Borisov captain Igor Stasevich's dangerous free kick.
Maurizio Sarri's reign at Chelsea already looks like it's approaching the end, and he's not even completed a season.
Sergio Aguero scored his 11th Premier League hat-trick as Manchester City humiliated Chelsea 6-0 at the Etihad.
Chelsea returned to fourth place ahead of Arsenal playing at defending champion Manchester City on Sunday.
From 0-0 at halftime, Chelsea conceded four goals to slump to its heaviest loss so far in Sarri's turbulent reign which makes it back-to-back league defeats for Chelsea.
The 26-year-old Spain international joined Atletico on an 18-month loan from Chelsea on Monday and was officially presented to the press on Tuesday.
Morata will return to the club where he started playing as a child. The 26-year-old Spaniard passed his medical exam on Sunday in Madrid.
The deal included an option for Chelsea to extend the loan by another season for 18 million euros ($20.5 million), or buy Gonzalo Higuain outright for 36 million euros ($41 million).
Sarri is the one deploying one of the world's most lethal wide forwards, Eden Hazard, as a center striker often playing with his back to goal.
Unai Emery's Arsenal, which lost last weekend to West Ham, made the most of an intense, attacking start — and Chelsea's lax defence.
The 36-year-old goalkeeper has a record 202 clean sheets in the Premier League and spent most of his glory days with Chelsea before moving to Arsenal.
Pedro Rodriguez collected David Luiz's pass before netting to put Chelsea in front in the ninth minute.
Kane, naturally, was delighted to score his 160th goal for Tottenham in the 26th minute, moving to fourth place on the club's all-time scoring chart.
In other all-English Premier League matchups in Monday's draw, Crystal Palace host Tottenham, and Manchester City will be at home to Burnley.
Fabregas has been linked with a move to French team Monaco, which is coached by former Arsenal teammate Thierry Henry.
Pulisic's contract with Dortmund ran until the end of next season.
While Vardy's lone goal saw Leicester hand Chelsea their first home defeat this season, Hodgson's Palace stunned Pep's City 3-2 to end their 100 per cent home record.
Napoli and Inter Milan will only have to cross the Italian border into neighboring countries for games against Zurich and Rapid Vienna, respectively.
Chelsea moved seven points behind first-placed Manchester City, which it beat last weekend to end the defending champion's unbeaten start to the season.
Although Chelsea failed to keep a perfect group-stage record, Willian and Giroud salvaged a 2-2 draw against Vidi in Budapest by scoring from free kicks.
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