City triggered their buy-back option in Angelino's deal to re-sign the Spanish fullback this week.
De Jong said, "Of course we would have liked to keep José for a while, but it is not strange that his huge development has been noticed by clubs at the level of Manchester City.
"There was a lot of interest in him. On the one hand we are disappointed that he left, but on the other hand it makes us proud, and we prove that we are preparing players for the absolute top here.
"We have deliberately agreed (to the clause) and after a year we have to conclude that José did a great job. Maybe even better than we had hoped. He has proven to City that raising the option is justified. He can be proud of that. We wish him every success in England."