The visitors looked on course for a hard-earned point at a rain-hit St Mary’s but Ireland international Long hit a quick-fire double in the final 15 minutes to condemn Nigel Pearson’s side to a fourth-straight defeat, which leaves Leicester in the relegation zone.
Southampton barely tested Kasper Schmeichel in the opening 75 minutes, despite dominating possession, but Long’s introduction proved decisive, as he curled in a Graziano Pelle lay-off with 15 minutes to go, before breaking through and slotting home a crucial second (85) – his first league goals for the club.
Sub Chris Wood could have set up a grandstand finish but wasted a golden opportunity on 87 minutes, chipping wide when clean through on goal, but Southampton fully deserved maximum points after a dominant performance, which moves Ronald Koeman’s side to within four points of table-topping Chelsea.
The hosts went into the contest full of confidence after recording the best defensive record and joint-top goal difference in the top flight from their first 10 games and immediately had Leicester on the back foot.
Dusan Tadic and then Morgan Schneiderlin both failed to find the head of Pelle with crosses into the box inside the first seven minutes, before the impressive Ryan Bertrand, afforded plenty of space on the left flank, whipped in a fine ball which Pelle really should have converted – although an offside flag spared the Italian’s blushes.
Nathaniel Clyne then rifled a shot into the arms of Kasper Schmeichel from the edge of the box after Leicester half-cleared a corner – but that would be Southampton’s only effort on target in the first period.
The best opportunity of the opening 45 fell to the hard-working Sadio Mane, though, and the Senegal forward should have worked Schmeichel from a tight angle, rather than blast over on 35 minutes.
At the other end of the field the returning Leonardo Ulloa was left isolated and only Jamie Vardy’s tenacity caused Jose Fonte any bother – the Englishman forcing Southampton’s captain to clear for a corner under pressure.
Steven Davis limped off just before the break – which will be a blow to Northern Ireland ahead of their European Qualifier with Romania – but his replacement Jack Cork was instantly up to speed with play, and made Esteban Cambiasso hack the ball behind to prevent Clyne from latching on to the substitute’s lay-off.
Leicester started the second period brightly, forcing an early corner, which was cleared by Pelle, but the Italian was soon causing the visitor’s backline problems himself and blasted a left-foot volley over the bar on 49 minutes after good work from Mane.
Southampton then had the ball in the net on 52 minutes, but Mane, who had slotted home a Bertrand centre, saw his effort rightly ruled out for offside.
With Wes Morgan proving to be a rock at the heart of the Leicester defence, clearing numerous crosses into the box, it seemed Leicester could be set to earn their first point in over a month – however, Long’s arrival changed the game.
Cork floated a cross into Pelle on 75 minutes and the Italian used his strength to hold off his marker and tee-up Long – and the forward curled a beautiful left-foot shot into the far corner from the edge of the box.
Leicester looked deflated and were hit again ten minutes later when Victor Wanyama lofted a ball through the middle, sending Long clean through on goal, and the Irishman made no mistake to double Southampton’s advantage.
Leicester could have forced a nervous final three minutes for their hosts had Wood found the target from 12 yards after breaking through Southampton’s backline – but the New Zealand international tamely clipped the ball wide with Fraser Forster exposed.
Long could have bagged a hat-trick with the last kick of the game but by then he had already won the match for Southampton, who continued their excellent run of results.
Phil Thompson’s verdict
Good teams play below par and still find enough to win the game. Kasper Schmeichel was not really worried in this game and he and Nigel Pearson would probably be feeling very happy with themselves going on 75 minutes – and then Shane Long comes on as sub and shows some great centre-forward play. This team is the Liverpool of last year. They play below par and they’ve still got enough to go and do it.
Defensively it is seven cleans sheets this season. That gives them the rock; a lot of teams defend poorly and this Southampton team defends better than most. The two centre-backs are excellent and the two full-backs can run like the wind. They get forward well and get back, tuck in and do everything right. That gives them a good base and then the three in the middle – Wanyama, Schneiderlin and Davis – all know each other’s place. They keep rotating to keep the ball moving. Tadic did not have his best game today, so his mum and dad will be disappointed after they flew over here to watch!