With Tottenham two points off top, HITC Sport looks at their transformation in a five-part series...
Any good team needs a good defence. It's one of the basic principles of football yet it's one so many choose to ignore.
At Everton, Liverpool and even Manchester City, so much more could have been achieved in recent times if not for a poor back four.
When the Argentine was brought to north London, he wanted to emphasise his high-pressing game and the need for quick, athletic players - also high in stamina.
Pochettino's demands are plentiful and not every defender can live up to them. That's why Tottenham conceded a fulsome 53 Premier League goals last term, more than any other side in the top 13 - and even more than relegated Hull City.
The likes of Vlad Chiriches, Federico Fazio, Younes Kaboul and more simply weren't good enough. Spurs needed a leader at the back and someone with genuine quality.
They already had Hugo Lloris and Jan Vertonghen, while full-backs Kyle Walker and Danny Rose have remained. But that second centre-back slot needed filling with a player worthy of the responsibility. This was where Tottenham had to look outward.
For a holding midfielder, Pochettino saw the potential in Eric Dier. But, for his new centre-back, the Argentine went for Toby Alderweireld.
The move made perfect sense. The Belgian was a countryman and former club team-mate of Vertonghen's. At £11.5 million, fee as reported by the Daily Mail, Alderweireld wasn't extravagantly expensive, either.
But little did Tottenham fans know just how good the 26-year-old was really going to be.
Several months on, it's a total mystery as to why Chelsea, Manchester City, Manchester United and others didn't move for the former Southampton loanee. And it's a big reason why Tottenham are above all of them.
Alderweireld has truly steadied the ship in defence. His partnership with Vertonghen has, at times, been telepathic but he has quickly formed a new one with Kevin Wimmer since the Belgian got injured.
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Alderweireld can tackle, he can read play, he is quick enough to chase the Premier League's top strikers and he can pass better than most defenders in the top flight - if not all of them.
"I was worried when Jan Vertonghen limped off [earlier in the season]," former Tottenham boss Tim Sherwood told Match of the Day 2 after the north London side's 2-1 win over Manchester City on Sunday.
"But Alderweireld for me is as important to Tottenham as [Vincent] Kompany is to Manchester City. Maybe the best defender in the Premier League."
It's not often a player can be considered a bargain at a price of over £10m. In Alderweireld, however, Spurs have just that - and a huge factor behind their title surge.