The negotiations only really got going early this afternoon, but by the evening, the deal had essentially been done.
According to Calciomercato.com and Sky Sport Italia, Atalanta and Milan have agreed the paid loan for one season with an option to buy.
La Dea will also pay most of his salary during the loan period.
De Ketelaere had many other options including Real Sociedad, Aston Villa and PSV Eindhoven, but he was determined to make his mark in Serie A.
It was a deeply disappointing debut campaign without a single goal, missing several big opportunities, following the massive €35m move from Club Brugge last summer.
De Ketelaere and Milan both hope that he can recover some confidence and form during the loan spell.