Brazilian teenager Gabriel Martinelli thought he had scored the winner when his goal was ruled out for offside seconds before Dembele put Lyon 2-1 up. Martinelli made his first appearance at Emirates Stadium as a substitute, with fellow new signing Dani Ceballos.
Pierre-Emerick Aubameyang’s volley had given Arsenal a first-half lead.
The Gabon international hit the outside of the post late in the second half, almost chipping Lyon keeper Anthony Racioppi from the edge of the box. Arsenal were forced to reshape early on as Alexandre Lacazette, facing his former club, was substituted in the 12th minute, moments after receiving treatment on the touchline.
Aubameyang’s volley in the 36th minute was cancelled out by Dembele’s first – a powerful header into the bottom right corner.And there was drama late on as Martinelli’s strike was ruled out seconds before Dembele went up the other end and slotted it past a stationary Bernd Leno to double his tally.