Southampton boss Mark Hughes had probably hoped a place in the Carabao Cup quarter-finals would alleviate pressure on his job with league performances far a few between, but Leicester City had other ideas.
Following a goalless draw, which saw Southampton had a goal ironically disallowed through VAR, Leicester won 6-5 on penalties to book their place in the next round.
Manolo Gabbiadini, who came on in the 90th minute, nearly played hero when his shot struck the crossbar — the Italian, however, then saw his penalty saved before Nampalys Mendy converted the winner.
Michael Obafemi and Yan Valery were each handed spots in the starting line-up, with Hughes hoping youth could spark a revival.
Obafemi, who has been scoring goals at will for the Saints Under-23s, was twice denied his first senior tally either side of the break.
The 18-year-old came closest in the second-half after Nathan Redmond’s effort hit the bar before falling to Obafemi, who could only side-foot the rebound wide of the post on 64 minutes.
The sparingly used Steven Davis would hit the back of the net in the 83rd minute, but VAR ruled that Redmond had handled the ball in the build-up.
Gabbiadini, who had been introduced at the start of stoppage time, had his free-kick saved off the bar by Danny Ward and the game was sent to penalties.
Leicester: Ward, Simpson, Evans, Soyuncu, Fuchs, Diabate (Albrighton 78), Silva (Mendy 61), Ndidi, Gray, Iheanacho (Okazaki 61), Vardy.
Unused subs: Jakupovic (GK), Morgan, Ricardo, Leshabela.
Bookings: Evans (90+6).
Southampton: Gunn, Targett (Cedric 62), Yoshida, Vestergaard, Stephens, Valery, Hojbjerg, Lemina, Armstrong (Davis 73), Redmond, Obafemi (Gabbiadini 90).
Unused subs: McCarthy (GK), Hoedt, Ward-Prowse, Austin.