The South Korea star made it five goals for the season - in fact, five in his last three away games - as we recovered from the disappointment of losing our opening match in the competition at Wembley a fortnight ago.
Erik Lamela sent Son clear in the 71st minute and he squeezed his finish through the advancing Igor Akinfeev.
It was no less than we deserved at Arena CSKA in Moscow as we controlled the game from start to finish and racked up another 22 attempts at goal (to the home side's eight).
Dele Alli came closest to breaking the deadlock in the first half when his rasper rattled the crossbar.