He had stooped to meet Troy Deeney's headed pass but the ball appeared to come off his hand to beat Southampton goalkeeper Alex McCarthy.
Meanwhile at Vicarage Road, Watford's Doucoure used his hand to score Watford's dramatic late equaliser - an incident that BBC Final Score pundit Kevin Kilbane called "an absolute disgrace". "[If] VAR comes in we probably wouldn't get that but today we got it". "It's the luck of the draw", Deeney told Sky Sports.
However, referee Roger East and linesman Derek Eaton either missed Doucore's action completely or were happy to let the goal stand as scored.
Had Southampton preserved their one-goal lead it would have been their first Premier League win in 10 matches.
"When you are winning 2-0, you expect your opponent to play very direct and it's something that's hard to defend because maybe they are a little bit better than us in this aspect".More news: Ethiopia to release all political prisoners
"I never see this type of mistake in the Premier League before".
A sheepish Marco Silva, the Watford boss, admitted: "I don't need to talk about the goal. It's something that can happen".
James Ward-Prowse scored Southampton's goals in the 20th and 44th minutes, with Watford jeered off by its own fans at halftime.
A long ball found Daryl Janmaat in the area and he crashed a shot off the crossbar.
But there was nobody there to save Silva's side when Long burst down the left to pick out Tadic, who in turn laid on a simple finish for Ward-Prowse.More news: 'Tide In, Dirt Out': Deadly Detergent Eating Flashmob Jumps to Dangerous Scale
Then came the late drama which prevented Watford from slipping to a seventh defeat in their last eight matches.
"I thought we played a really good first half". "It's true we did well in the second half".
A week in which VAR technology was used on trial for the first time in English club football to a mixed reception ended on Saturday with Premier League managers calling more urgently for its introduction after two highly contentious decisions. We showed we can be risky in open play on the counter-attack, and in defence for most of the game we played well.
Amid speculation some sort of VAR could be in place at this year's World Cup in Russian Federation, the problems in France included both a failure to make a referee's watch vibrate to confirm a goal had been scored and the reverse issue, when the match official's watch vibrated when the ball was held by a goalkeeper well before it had crossed the line.
"My assistant says it's a very clear penalty and a red card also, so it's another decision that creates something important but what can we do?"More news: Chevrolet Silverado 1500 Revealed in Detroit
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