A torrid four-game stretch, in which Tottenham crashed out of the UEFA Europa League and saw their Premier League top-four aspirations left in tatters as a result of defeats to Chelsea and bitter rivals Arsenal, preceded Southampton's visit to White Hart Lane.
That left all eyes on Sherwood, for whom happy memories of his first win at the helm – in the reverse meeting with Mauricio Pochettino's men – seemed increasingly distant.
The Tottenham head coach's misery was compounded in the early stages as two Kyle Naughton errors paved the way for Jay Rodriguez and Adam Lallana to put Southampton 2-0 ahead.
Yet Tottenham showed spirit thereafter and Christian Eriksen levelled matters with goals either side of half-time.
The turnaround was completed in the most dramatic fashion as, with two minutes of injury time to go, a powerful half-volley from Sigurdsson left Artur Boruc with no chance.
There were signs of life from Sherwood's men on Thursday as they threatened a stunning turnaround before ultimately losing 5-3 on aggregate to Benfica in Europe's secondary competition.
Yet any confidence gleaned from that performance was not in evidence during an open start as Tottenham afforded their opponents too much space – although that was not the contributing factor when the visitors were gifted an opener.
Naughton completely misjudged the simplest of headers from Boruc's goal kick, paving the way for Rodriguez to race clear and curl home his third goal in as many games.
Things quickly went from bad to worse for Naughton, too, as another failed clearance fell to Rickie Lambert, whose simple ball enabled Lallana to slot past Hugo Lloris.
Nathaniel Clyne then continued the theme of errors from right-backs, allowing Tottenham back into the game.
He completely fluffed his lines attempting to clear Nabil Bentaleb's cross and Eriksen was on hand to side-foot past Boruc.
The frenetic nature of the game showed no signs of halting before half-time as Rodriguez and Eriksen both spurned good chances.
A break did little to dull proceedings either, with Tottenham equalising within a minute of the restart.
Roberto Soldado stole the ball from Dejan Lovren on the right wing, cut inside and slid across to Eriksen – who made no mistake with the goal gaping.
Southampton completely lost their early verve as Tottenham piled forward with increased fluency in search of a third.
Nacer Chadli hit the side-netting, but it appeared as though a point would have to suffice for the outspoken Sherwood, only for Sigurdsson to win the game with a stunning intervention.
