Messi was taking part in the FIFA World Youth Championship in the summer of 2005 while Rijkaard, then Messi's manager at Barcelona, and de Boer were holidaying in the Caribbean.
De Boer felt the weight of expectation was weighing the 17-year-old South American superstar down and he was losing focus trying to play 10 different positions.
Rijkaard disagreed and completely ignored his advice and after Messi inspired Barca to three La Liga titles and two Champions League triumphs de Boer reckons he might have called it wrong.
He told the Daily Record: "I remember going on holiday with Frank Rijkaard to Aruba and we watched Messi playing in the FIFA Youth Championship.
"Frank kept raving about him because he was already at Barcelona but I didn't think he played a great tournament.
"I told Frank that he was trying to do too much and wasn't focused enough.
"I said he needed to play more as team player. Frank said if he just told him to stay in the penalty box it would restrict him.
"He told me Messi had to play with his instinct and that if you try to limit him then you take away what is so special.
"Frank worked with him daily so had much knowledge of his capabilities. That's why he was so convinced the boy was something special.
"I think we saw that against Arsenal although I was not surprised by his performance against as I have seen him play even better games."
De Boer feels Ronaldo was a better player at the same age but believes he has already surpassed fellow countryman Maradona.
He said: "Messi has already achieved much more than Maradona did at the same stage of his career. He is so influential for Barcelona and Argentina and keeps on winning the big prizes. There is no limit to what he can achieve and if he can win a World Cup he can lay claim to being the best there has ever been.