The Reds signed the Netherlands international in a world record fee for a defender from Southampton in the January transfer window.
Van Dijk was rated as one of the top defenders in the Premier League prior to his move but had only previously played for Celtic and Willem.
Alderweireld, on the other had, had helped Spurs to secure successive top-four finishes and was described as the league’s best centre-half.
Alderweireld’s current deal will expire at the end of next season but Spurs do have the option to extend the Belgium international’s contract by an extra 12 months.
However, if Spurs do opt to activate the clause in Alderweirld’s contract then the 29-year-old would be available for a cut-price £25m deal in the 2019 summer transfer window.
Sunday Times reporter Castles believes Spurs won’t sell Alderweireld for anything less than Van Dijk’s £75m fee.
“The asking price has been £75m on Tottenham’s part,” Castles told the Transfer Window podcast.
“It’s the fee Southampton secured for Van Dijk. You can see Levy’s logic.”
Spurs finished in third place in the Premier League table last term and one spot adrift of their rivals Manchester United.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news