Spurs fans were left not knowing whether to laugh or cry.
Harry Kane, however, could be out for the season.
The absence of the in-form England striker would be a hammer blow to the League and Cup ambitions of Mauricio Pochettino’s high-fliers.
Yet If the nightmare scenario is confirmed over the next few days, the South Korean’s scintillating form here at least gives Spurs a lifeline.
Son’s treble was a stunning riposte to the idiots in the away end who targeted him with chants of ‘DVD’ and ‘Three for a fiver”.
Hilarious. Tottenham’s £22million forward lampooned as the bloke who sells DVDs down the market.
Or yet another example of fans who believe there is nothing wrong with mocking a player because of his ethnicity. Critics of the Spurs’ fans description of the Millwall supporters as ‘Pikeys’ would be entirely justified. The FA would be right to take a look at matters off the pitch here.
On the pitch Son had the last laugh. Three for a fiver? Son’s answer came in the shape of three in Spurs six-fer.
His strikes were also a reminder that Tottenham have others who know where the goal is.
It looked bleak when Kane limped off after just eight minutes. He’d blasted an effort at keeper Tom King and, as the Millwall rookie beat it out, defender Jake Cooper’s challenge left Kane on the deck.
White Hart Lane held its breath and the worst fears of the home contingent were realised when the highly-prized marksman signalled he couldn’t continue. Pochettino later confirmed he had injured the same ankle (the right one) that had kept him out for seven weeks during the first half of the season.
Back then Tottenham struggled, crashing out of the Champions League because of their lack of firepower.
Here they were simply too good. Christian Eriksen and Dele Alli also filled their boots while even Vincent Janssen weighed in for his first goal from open play in 30 attempts since signing for £17million last summer.
Millwall fan KOs Spurs supporter after FA Cup thrashing
The question now is whether they can all continue producing on a regular-enough basis for Spurs to keep dreaming of more Champions League football and winning the FA Cup.
Eriksen opened the scoring with his second goal in his last four games, a first-time strike into the bottom corner on 31 minutes as Alli was attempting to bring the ball under control.
Son curled a magnificent second beyond King, in for the injured Jordan Archer, ten minutes later.
On 54 minutes, Son hit a stylish second, Tottenham’s third, from just inside the box on the volley.
Millwall, undefeated in all competitions since December 17, were punch drunk by now. Their impressive defensive record – one conceded in their last ten – was already in pieces.
Neil Harris’s team had previously seen off three Premier League clubs but this particular step up in class was proving a bridge too far.
Worse was to come. Alli rammed in Eriksen’s ball across the six-yard box to make it four with 18 minutes left. Then substitute Janssen – the man who had seemingly forgotten how to score from open play since his arrival from Alkmaar last summer – got in on the act.
Son shaped as if to burst into box but instead squared it to the Dutchman who hit it first time into the bottom corner.
Fittingly the last word lay with Son. Erikssen raced down the left, crossed into the box and King looked to have held Son’s first-time volley.
Instead the 22-year-old fumbled, the ball bounced through his legs and over the line for 6-0.
The FA Cup semi-final berth is a just reward for Tottenham’s impressive campaign. You’d have fancied them for more, however, with Kane – scorer of 14 goals in his previous 12 – fit.
Without him you’d worry. It is turning into an anxious end to the season in both sides of north London.