The 21-year-old Danish international is currently in talks with Tottenham Hotspur, although a fee is yet to be agreed.
"His departure will be an enormous loss," De Boer told De Telegraaf. "Chris was very important for us.
"Players who are that good and also the ideal professional are, for a coach, the most ideal players."
When asked if he thought that Eriksen would succeed in the Premier League, De Boer replied: "It would surprise me if that won't happen."
Eriksen has already found the net twice for Ajax this season, including the equaliser in last Friday's 3-3 draw at Heerenveen.