The Dutchman has completed a permanent move to Girona in La Liga for a modest fee.
The United website officially confirmed the departure of the Netherlands midfielder.
Posting on Instagram, Donny sent this message: "Dear Manchester United family, today is a memorable day as my adventure at the club comes to an end.
"I want to thank you all for your support throughout the years. From the moment we arrived at Manchester United, we felt extremely welcome and we are grateful for that.
"Thank you to everyone at the club for making us feel at home. A special thanks to all the fans that made these years unforgettable.
"Besides the football, the birth of our two children here ensures that Manchester will always have a special place in our hearts.
"Good luck to everybody, we'll meet again."