The midfielder left Ajax for Newcastle last week.
De Boer said: "We were able to enjoy one more year with Siem.
"I do not begrudge him. He is a great example of the new talents. He was always there for everyone, a real team player."
De Boer added: "I was expecting to leave. I was told he was likely to leave, so I was prepared for it."
For the big stories and the banter follow us on Twitter: @tribalfootball