Long almost had a second less than seven minutes later but his angled volley was pawed away by Heurelho Gomes before Sadio Mane had the chance to make it 2-0 in the 36th minute only to fire his golden opportunity off the upright when one-on-one with the Hornets keeper.
With just over 15 minutes remaining, Southampton doubled their lead when substitute Dusan Tadic raced onto Steven Davis' header, worked his way around Sebastian Prodl and Ikechi Anya, and fired beyond Gomes to seal the match for the Saints in keeper Fraser Forster's first team return following a lengthy absence with a knee injury.
The win sees Koeman's men move up to 12th position on the table where they sit just two points behind Quique Sanchez Flores' Watford who have now lost three in a row and have not won in the league since before Christmas.
2015/16 BARCLAYS PREMIER LEAGUE - SEASON SCORES
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