Shortly after Mohamed Salah broke the deadlock midway through the second half, Alexander-Arnold stepped up to take control of a set-piece situation 25 yards from goal.
The No.66 produced an unstoppable shot, swerved over the Watford wall and into the top right corner to double a lead that was later added to by Roberto Firmino at Vicarage Road, as the Reds increased their unbeaten run in the league to 13 games.
Post-match, Alexander-Arnold explained how he came to be the man with the responsibility at the free-kick – and then use it in sublime fashion.
“Before the game, me, Hendo, Millie, everyone was joking about who would take it,” he told Liverpoolfc.