The centre-back also scored the winner in his side’s Champions League quarter-final victory over Juventus, sending the Eredivisie winners to the last four of the competition for the first time since 1997.
And Capello told Corriere dello Sport that Juventus would ease their way to a ninth straight Scudetto should they land their target.
“If Juve sign De Ligt then it’s over. Again. And we go directly to the season after," he said.
“De Ligt is the best defender out there. He allows you to defend with two, freeing up the full-backs, like he has at Ajax.
“With him, Juve will have hit the jackpot. Without him, well, it could help the others get closer.
Napoli finished second in 2018-19, 11 points behind the champions, but Capello thinks it will be much tighter if they sign Roma defender Kostas Manolas, with whom they have been heavily linked with in recent weeks.
“I’ve read about Manolas to Napoli. With him and Kalidou Koulibaly, as well as a couple more quality reinforcements, Napoli become very competitive.”