"Gareth Southgate said he wanted to go with younger players and build for the next five years and beyond," said the former Dutch international striker.
"I know the club has produced, and is producing, a lot of good youngsters and Gareth wanted to give them an opportunity.
"He explained to me he also wanted to bring in his own players. That is understandable. It's what every good manager does and I respected him for his honesty.
"We didn't achieve everything we wanted to while I was there, but we did come very close to doing some great things. I have been very fortunate and played at some great clubs during my career and Middlesbrough is right up there with them.
"I have never had any regrets about the two years I was there. I like to think I learnt a few things as well and the club helped to improve me as a player.
"I would be lying to say it would not be special to score at the Riverside, especially as we at Cardiff are just as anxious to be in a cup semi-final as Middlesbrough."