On an evening when Watford were as poor as the hosts were intense, goals in each half from Shane Long and Dusan Tadic took Southampton into 12th place and within two points of their underwhelming visitors.
Southampton were the more energetic of the two teams immediately after kick-off and Matt Targett, playing as a wing-back, received possession on the left wing before curling an accurate, 17th-minute cross into the penalty area towards Long, from where the forward neatly headed beyond goalkeeper Heurelho Gomes and inside the right post.
Watford’s response was to become even more sluggish, and erratic, and to allow Sadio Mane to run near-unchallenged from the left wing and to the edge of the penalty area before shooting and watching Gomes palm his effort wide.
The hosts otherwise remained in control but the concern remained that with Watford’s talent a one-goal lead would not be enough, and had they secured an equaliser, Mane — more than any other — would have felt responsible. After intercepting a rare misplaced pass from Jose Jurado, the forward found himself immediately in front of goal but instead struck the outside of the post.
Pressure had been building on Koeman in recent weeks, but as if to highlight Southampton’s poor run had reached its conclusion and that he remained firmly in control, the manager substituted Mane for Dusan Tadic and was almost instantly rewarded with a goal.
Controlling possession after Davis’ header, Tadic ran into Watford’s penalty area, created space, and classily finished low beyond Gomes from in front of goal in the 73rd minute.
Forster, Cedric Soares (Yoshida 82), van Dijk, Fonte, Bertrand, Romeu, Clasie, Steven Davis (Pelle 81), Mane (Tadic 71), Targett, Long.
Gomes, Nyom (Ibarbo 68), Prodl, Britos (Cathcart 72), Holebas, Abdi (Anya 61), Capoue, Watson, Jurado, Ighalo, Deeney.