The 21-year-old's contract with the Dutch club is due to expire next summer and Spurs have entered negotiations for the attacking midfielder, with reports placing the fee at around €10 million (£8.6m).
De Boer expressed his sadness as seeing the midfielder depart but says that the ultimate agreement from the Dane himself is all that remains to be completed.
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The north London club completed deals to bring in Erik Lamela and Vlad Chiriches on Friday with money freed up from Gareth Bale's impending transfer to Spanish giants Real Madrid.
Andre-Villas Boas's interest in Eriksen is set to make him the seventh signing for the club this summer and De Boer sees the sale as "an enormous loss".
"His departure will be an enormous loss, Chris was very important for us," the Ajax boss told De Telegraaf.
"We always have work in a good way. Players who are that good and also the ideal professional are, for a coach, the most ideal players."