The Belgium international, thought to be on the radar of Arsenal, Manchester United, Tottenham and Newcastle, is out of contract in Amsterdam next summer.
The defender said earlier this month he is yet to discuss a new deal and hinted the Dutch club could let him go at the end of the season.
De Boer says Ajax will open talks with the centre-back over an extension in March but insists the club will not risk losing him on a free transfer if he rejects the offer of a new deal.
"We know that his contract expires in 2013," he told Eredivisie Live.
"So it's either renew or relocate to a good foreign club, where Ajax would also benefit from a transfer fee.
"We already told Jan last year in Turkey that we are keen to hold on to him. We will open talks early in March to discuss a new deal."
