The former France international spent three years playing alongside the Argentine between 2007 and 2010 and said that he witnessed the No.10 do things that he did not previously think possible.
"You cannot define him in words," Henry told El Mundo Deportivo. "He's from another world; the best.
"To have him beside you every day is tremendous. He does unimaginable things. I do not know just how far he will be able to go in his career, but he's definitely already an incredible player."
Henry revealed that he also loved his time under the tutelage of former Barcelona coach Pep Guardiola, explaining that the Catalan proved nearly as big an influence on him as Arsenal boss Arsene Wenger.
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"But I must say that Pep proved surprisingly spectacular. From a strictly business point of view, he is the best boss that I've had because he gave me discipline and a proper view of the game.
"Everything was planned; he left nothing to chance and was always one step ahead of the others.
"Pep and Wenger are my two reference points [in terms of coaching]; both are extraordinary."
Turning his attention to Barcelona's Champions League last-16 tie with Manchester City, Henry is hoping that the Blaugrana prevail but freely admits that they are going up against a strong side.
"It will be difficult [for Barca]," the New York Red Bulls striker conceded. "Manchester are better than ever this year.
"They have made a very strong investment [in their squad] and are showing their full potential in the Premier League.
"But I also think that having a rival like this will work well for Barcelona because City like to play without closing things up at the back. So Barcelona should be able to attack as they please.
"I'll watch the game on TV and I'll be cheering on Barca like a fan."