Pellegrino sent on Mario Lemina as he looked to stem the tide and Southampton did initially stabilise, helping to quieten the expectant home crowd until Doucoure had the final say, turning in Deeney's header across goal and evading the attentions of referee Roger East.
It was possible to make a case for saying Doucore's action had been inadvertent rather than deliberate but not as far as Martin Keown was concerned, with the BBC pundit saying the Watford man had "done a (Diego) Maradona" - a reference to the football great's blatant handball that helped Argentina knock England out of the 1986 World Cup on the South Americans' way to lifting the trophy.
"I understand that many people would like to see Doucoure face retrospective action for deceiving the officials but that won't happen".
Watford 2-2 Southampton
"If VAR had come in, we probably wouldn't get that but today we got it".
"I never see this type of mistake in the Premier League before. It's the luck of the draw", Deeney told Sky Sports.
Watford boss Marco Silva added: "It's not an easy moment for the opposition but decisions can go against you".
Southampton, who kicked off only outside the relegation zone on goal difference, were on the verge of a much-needed win after two first-half goals from James Ward-Prowse.More news: Coast Guard rescues 5 fishermen in Monroe County
The midfielder's first goal came in the 21st minute as his deft touch found the back of the net, after Shane Long found him with a low cross just inside the area.
Andre Gray pulled one back before the hour when he bundled in the rebound after Daryl Janmaat's shot was tipped on to the crossbar by Alex McCarthy.
The striker raced 60 yards along the touchline to beat Molla Wague to a clearance and crossed for Dusan Tadic, who laid the ball off for Ward-Prowse to fire home.
"We didn't play with the right mentality or attitude in the first half".More news: Fiat Chrysler to invest over $1 billion in Ram Truck plant
Watford have conceded 25 goals at Vicarage Road in the Premier League this season - the most of any side in home games.
"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: Everton vs. Tottenham: a pre-match word from Mauricio Pochettino