Bad news for Spurs as it looks like the defender hurt himself during that attack and will need to be replaced.
Destiny Udogie prevents Southampton an attack, drives down the pitch and finds Son on the left, who squares the ball for Pape Sarr to wriggle through a weak defence before stabbing home.
"This is target practice for Tottenham," says TNT Sports commentator Darren Fletcher as Southampton fans begin to leave the stadium in their droves. We're only 25 minutes in!
Despite being three goals down, the hosts have still hardly been out of their own half.
It looks like they've switched to three at the back.
It's another one! Spurs have a third after more awful Southampton defending!
Son crosses from the left to Dominic Solanke in the six-yard box, who is challenged but the ball falls to the unmarked Kulusevski who slams the ball home from close range.
A deep cross from the right evades three red and white shirts in the box and falls to Son on the far side, who makes no mistake with a low finish which fizzes past McCarthy.
The ball drops to Spence from the set-piece which is blocked and the visitors recycle the ball, find Son on the left who cuts in and sends an effort straight at McCarthy.
Spence looks sharp down that right-hand side too. They look up for this, Spurs, but we all know they will give Southampton a chance. Can they build on their sharp start?
Spence drives forward from right-back and threads a pass through to James Maddison who slots beyond Alex McCarthy for his sixth goal of the season.
"Another performance of ours that reflects the kind of football we want to play," the Australian told TNT Sports.
MARTIN: 'SOMETHING MISSING IN BOTH BOXES'
And both managers have been speaking to TNT Sports, with Saints boss Martin telling us what he wants from his group tonight...
"Lots of things. I think the mentality of the team is the most important thing tonight. There has been so much to like about a lot of our performances and then something missing in both boxes, really.
"Hopefully a little bit of ruthlessness, a bit of aggression and a bit more intent."
Speaking of managers under pressure, Gary O'Neil is the latest Premier League boss to lose his job, after Wolves were beaten by Ipswich on Saturday.
Is the four-time champions Manchester City, who have just lost for the eighth time across their last 11 fixtures.