The Dutch midfielder also admits he was fortunate not to have been sent off by referee Howard Webb in a match that featured a record 14 yellow cards and one red to the Netherlands' Johnny Heitinga.
During a spiteful World Cup final, won by Spain 1-0, De Jong jumped in the air and kicked Alonso in the chest with his studs that left him seeking sideline treatment before playing out the match.
Arriving back in Spain on Monday with his jubilant team-mates, Alonso left no-one in any doubt as to his thoughts on the tackle.
"It was one of the worst tackles I have ever suffered," he said.
"It was a tight game, there was respect from both sides and they had to close us down.
"But that challenge was really hard. It was one of the most painful tackles in my life and it was hard to play on.
"I have probably broken a rib, although I am not too worried about that."
De Jong admitted he was lucky not to have received a red card for his challenge.
"Yes, I was concerned it might be worse than a yellow," the Manchester City midfielder said.
"It looked worse, although to be honest I didn't see the opponent coming in from the side.
"I was really focused on the ball and I caught him on his chest.
"It was a bit curious but he gave the yellow card so for me it was a little bit of luck."
The Dutch were scathing in their attack of Webb after the match, with coach Bert van Marwijk claiming the Englishman lost control of proceedings.
De Jong, though, was far more complimentary.
"It is easy to blame the referee," he said.
"There were some strange decisions and everyone, in the stadium and on TV, could see we should have had a clear corner when he gave a goal-kick to Spain. On the next attack they score. What can you do about that? He will realise what he did when he sees the replay.
"But I am not saying the biggest fault was with the referee.
"I know him from the Premier League and have never had a problem with him.
"He refereed the Champions League final at the end of the season so he has a lot of experience.
"I like the referees in the Premier League. Howard is not a bad referee. But sometimes you don't have your luckiest games. Unfortunately, in the final, he did not have his luckiest game."