Ralph Hasenhuttl received a sobering lesson as to his tough task at Southampton as Jannik Vestergaard's error gifted Cardiff City a 1-0 home win on Saturday.
Callum Paterson was the beneficiary of a poor Vestergaard backpass that condemned the Saints, who sacked Mark Hughes following last weekend's draw with Manchester United, to a seventh loss in 11 Premier League matches.
The visitors had looked to have steadied following a first half in which they rode their luck for long periods, Alex McCarthy making several important stops.
But there was little the goalkeeper could do to atone for his team-mate's game-changing mistake as ex-RB Leipzig boss Hasenhuttl was left to ponder how best to fix a defence that continues to undermine their bid to escape the drop zone. Southampton are three points adrift of safety, while Cardiff are now four points clear of the bottom three.
Cardiff were on top from the outset and would have been ahead early had Harry Arter connected cleanly with an inviting volley.
Mario Lemina was no more clinical in the opposite box, the midfielder slicing over when Paterson's defensive header from a corner looped into the air and rebounded kindly off the crossbar.
McCarthy, having twice foiled Josh Murphy, was called on once more before the break to tip over a thunderous Nathaniel Mendez-Laing drive.
Southampton improved after the restart and were aggrieved not to have been awarded a penalty on the hour, Victor Camarasa's outstretched leg appearing to make slight contact with Stuart Armstrong.
A contest short on attacking quality always seemed likely to be decided by a mistake and so it proved in the 74th minute, Vestergaard selling McCarthy short under scant pressure and enabling makeshift striker Paterson to seal his side's fourth home win in five fixtures.
It is perhaps most troubling for Hasenhuttl that Vestergaard's mistake came during a relative period of comfort for Southampton, highlighting just how the Saints' rarely settled defence is capable of coughing up cheap goals at all junctures. Investment in January appears the Austrian's best route to solving an ongoing issue.
Morrison crucial to essential clean sheet
He might well have grabbed himself a goal, but Sean Morrison was no less a key factor in Cardiff's first clean sheet since August. The centre-back was a constant threat in the opposition box and dealt capably with Charlie Austin, making a team-high nine clearances.
Vestergaard costs new boss
Dropped for the midweek defeat at Tottenham, Danish defender Vestergaard - who, like Hasenhuttl, arrived at Southampton by way of the Bundesliga - might soon find himself back on the bench. Paterson had given up chasing Camarasa's poked pass prior to the lame backpass.
Key Opta Facts
Cardiff have won four of their last five Premier League home games (L1), as many as they had won in their previous 22 in the competition (W4 D6 L12).
Southampton's current 14-game winless run is (W0 D7 L7) is their longest such-run in all competitions since March 1989 (20 games).
Cardiff ended a run of 12 Premier League games without a clean sheet, since a 0-0 draw with Huddersfield in August.
Southampton are winless in their last nine Premier League games against newly promoted teams (D5 L4).
Southampton have lost eight of their last nine away league games against Cardiff (W1).
Nine of Callum Paterson's 14 league goals for Cardiff City have arrived on home soil (64 per cent).
Cardiff will hope for an improvement in away results when they head to Watford next Saturday, while Hasenhuttl's first home fixture is a testing date with Arsenal the following day.