Match ends, Southampton 1, Watford 0.
Southampton boss Mauricio Pellegrino said his side's FA Cup fourth round win over Watford shows they are "growing and improving as a team."
A birthday goal from Jack Stephens condemned new Watford coach Javi Gracia to defeat in his first game.
Stephens converted from close range to provide some relief for the hosts who are winless in 11 league games.
Injury-hit Watford struggled to create but nearly forced a replay, Christian Kabasele heading wide in added time.
Dane Pierre-Emile Hojbjerg hit the crossbar from 30 yards for Southampton.
Striker Shane Long also missed a string of chances to ease the nerves of the Saints, who brought on new £19m striker Guido Carrillo for his debut as Pellegrino looked to kill off the tie.
Gracia also turned to his bench, sending Stefano Okaka as he attempted to earn a replay. Okaka sent an overhead kick over the bar, but Southampton keeper Alex McCarthy had a quiet afternoon.
It had been a scrappy contest but gives under-pressure Southampton more evidence of an upturn after draws with Arsenal, Manchester United and Tottenham during December and January.
Goals still remain an issue, Charlie Austin, their top scorer with six, is currently injured, so hopes will be high for ex-Monaco striker Carillo who had minimal impact during his 10-minute introduction to English football.
Two weeks ago in the Premier League, Southampton led 2-0 against Watford at Vicarage Road only to draw 2-2 when Abdoulaye Doucoure levelled in the 90th minute.
This time Saints held on for the win and Pellegrino said: "We are growing and improving as a team.
"We were working a lot to try and improve with the ball a little bit more. Every single game is an opportunity to believe that we can recover our level, our real level, and I think that today in some moments we showed that we are a little bit closer.
"The feeling in the dressing room is that we are in a good way. We are working hard and the mentality of the players is much better now.
"We have to be humble because in football everything can change suddenly but we have confidence and believe it's the right way."
Gracia must restore unity
Gracia spoke warmly of Watford's family feel beforehand and one of his tasks now is to restore unity on and off the pitch.
The Hornets appointed the Spaniard, swiftly after sacking Marco Silva on 21 January, with the club just five points above the relegation zone and amid reports that some of his players had lost faith in the Portuguese.
At the end of this latest defeat, there were angry exchanges between defender Jose Holebas and some fans in the away end.
Gracia, who left Russian club Rubin Kazan in June after a 10-year coaching career in his homeland and in Greece, can start soothing supporter frustration by improving his side's attack.
They managed just one shot on target all afternoon, from Andre Gray, but without several first-team players - captain Troy Deeney, goalkeeper Heurelho Gomes and Tom Cleverley among others - he had an uphill task at St Mary's.
Gracia called this game "a final" in the build-up, insisting "you only get one chance to go on in this competition" but after just five days of training with his new players it seemed his final third instructions were not yet ingrained.
Holebas had a good game, having more touches than any team-mate and making seven good crosses. His reaction at the end at least proved he cares.
If Gracia can channel that frustration in a positive way and revive his team's attack, he can hope for the new manager bounce to turn things around.
"I do not understand what supporters sing," he said of the chanting and criticism by Watford fans at the final whistle. "I only can say at the end of the match the players went to say thank you, and me too.
"I am very grateful and I feel sorry for them.
"Our players are in a difficult moment and need to work a lot but they need more confidence also.
"The only way to get that confidence is working, working and working. If we get a good result in our next match it's possible they feel better."
- 2Cédric SoaresBooked at 62mins
- 21BertrandSubstituted forPiedat 40'minutes
- 11TadicSubstituted forCarrilloat 83'minutes
- 19BoufalSubstituted forYoshidaat 67'minutes
- 2JanmaatSubstituted forSinclairat 74'minutes
- 23WatsonBooked at 61mins
- 29CapoueSubstituted forOkakaat 45'minutes
- 28CarrilloSubstituted forPereyraat 57'minutes
- Robert Madley
- Shots on Target
Second Half ends, Southampton 1, Watford 0.
Attempt missed. Christian Kabasele (Watford) header from the right side of the box is close, but misses to the right. Assisted by José Holebas with a cross.
Foul by Maya Yoshida (Southampton).
Stefano Okaka (Watford) wins a free kick in the attacking half.
Foul by Jérémy Pied (Southampton).
Abdoulaye Doucouré (Watford) wins a free kick in the defensive half.
Jérémy Pied (Southampton) wins a free kick on the left wing.
Foul by Jerome Sinclair (Watford).
Pierre-Emile Højbjerg (Southampton) wins a free kick in the attacking half.
Foul by Ben Watson (Watford).
Attempt missed. Stefano Okaka (Watford) right footed shot from the centre of the box is too high. Assisted by Roberto Pereyra with a cross.
Foul by Shane Long (Southampton).
Jerome Sinclair (Watford) wins a free kick in the defensive half.
Attempt blocked. Richarlison (Watford) right footed shot from the left side of the box is blocked.
Substitution, Southampton. Guido Carrillo replaces Dusan Tadic.
Foul by Shane Long (Southampton).
Christian Kabasele (Watford) wins a free kick in the defensive half.
Corner, Watford. Conceded by Jack Stephens.
Attempt missed. Jerome Sinclair (Watford) left footed shot from the right side of the box misses to the left. Assisted by Richarlison.