The midfielder joined the Black Cats on deadline day.
"The operation was not done overnight. Three times we refused the departure of Didier Ndong for offers from 12 to 14 and 16 million euros.
"But when the last offer of 20 million euros has arrived, maybe more with bonuses, his departure became inevitable for several reasons," explained Fer.
For Sunderland, the €20 million paid for Ndong was a club record.