Three minutes later, he turned provider as he teed up Shaqai Forde to double the Hornets' advantage.
Watford's Derek Agyakwa was shown a red card in the second half after denying Dylan Stephenson a goalscoring opportunity.
But the Magpies were unable to muster a response, with Charlie Wiggett heading their best chance over the crossbar.
Saints went in front through Caleb Watts' volley, and although Mika Biereth equalised, the Gunners couldn't find a winner.
Elsewhere, Exeter City booked their place in the group stage with a 2-1 victory over Plymouth Argyle in the qualifying round.
Arsenal 1-1 Southampton | SOU report
Middlesbrough 1-0 QPR | MID report
Nott'm Forest 4-1 Blackburn | NFO report
Watford 2-0 Newcastle | WAT report | NEW report
Mansfield 3-0 Sunderland | MAN report | SUN report
Southend 1-3 Fulham | FUL report