The Dutchman has been tipped for a switch to the Saints all summer.
The towering centre-back has been long-linked with a move to the Premier League and the Hoops' failure to secure a Champions League spot last week all but confirmed the Dutchman’s departure.
Celtic were determined to received over £10 million for one of their key assets and appear to have got their wish, with the Saints prepared to pay £11.5 million for his services.
Sky also report that the 23-year old underwent his medical last night and that would just leave the formalities to be concluded today.
The departure of the Parkhead fan-favourite will be a tough one to take for the Celtic faithful, but the capture of Tyler Blackett and the imminent arrival of Jozo Simunovic – as reported by the Scottish Sun – will help ease the pain.
Blackett arrives on a season-long loan from Manchester United while highly-rated Croatian Simunovic is set to complete a permanent switch from Dinamo Zagreb.