Cyle Larin scored with his first touch for Southampton to bag the only goal in a 1-0 victory over out-of-form and managerless Watford.
The Canada international completed a loan move from La Liga outfit Mallorca on deadline day and made an immediate impact with a booming header.
It allowed Southampton to win back-to-back matches for the first time since early December and build the foundations of a potential play-off push.
For the Hornets, who were led by interim Charlie Daniels for the second time, they are winless since New Year's Day and haven't scored in their last three matches.
A largely dour first half was punctuated by the odd moment of quality.
Mamadou Doumbia caught Southampton goalkeeper Daniel Peretz napping in the second minute, but Edo Kayembe couldn't take advantage as his shot was blocked.
Malian striker Doumbia then swung his leg at a knocked-down corner but failed to wrap his foot around it enough to find the right side of the post.
Watford only hit the target with one of their six shots in the opening 45 minutes, when Othmane Maamma fired straight at Peretz after Giorgi Chakvetadze had cut the ball back to him.
The Georgian then lashed over after finding himself in space on the left side of the penalty area as the Hornets failed to properly test Peretz.
Saints had half the number of chances, but the best efforts fell the hosts' way.
Ross Stewart forced Egil Selvik into an excellent reaction save after the ball had fallen loose for the Scot in the box.
Leo Scienza livened up an uneventful beginning to the second half by first sending a free-kick just over the bar.
Then the Brazilian outpaced the visiting defenders and aimed for the bottom corner, but Selvik made a second exceptional save of the match.
Marc Bola was a tiny deflection away from giving Watford the lead but his driven shot fizzed narrowly wide.
Saints boss Tonda Eckert decided it was time to freshen up his attacking line in the 67th minute - bringing on new boy Larin and Sam Edozie, who was making his first appearance of the season having been frozen out.
Within three minutes, Saints were ahead, as Larin beat his man to head in James Bree's corner - his first touch for the club.
After that, the home side locked up their defence and only gave up two shots - neither of which tested Peretz.
Southampton's Tonda Eckert:
"We knew with Cyle we would have the physical presence we need. He scored the goal but it was also the way he defended at the other end.
"I think it couldn't go any better than it did for him. Strikers live off those moments.
"The nice thing with Cyle is that he has three kids and has lived in a lot of different countries, so it has made it easier for him to come into the group.
"He has a proper presence. He is a proper family man and a father, with the experience he has collected over the years.
"It has been very easy for him to find his spot in the changing room and combining that with a nice performance has made it even easier."
Watford's interim boss Charlie Daniels:
"I thought it was another good performance, especially in the first half.
"Our levels dropped in the second half which allowed Southampton to come back into it, but we created loads of chances but couldn't put the ball in the back of the net.
"Goals are the main thing in the game. We created a load of chances but we didn't work the goalkeeper enough.
"The players know that, I told them after the game. We talk about accountability. That doesn't just mean out of possession and doing the dirty work, but also being accountable when you get a chance.
"We have to keep working hard to change that and repetition. Hopefully the next moment they get they will score."
