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Over the course of his glittering career, the six-time Ballon d'Or winner has regularly received praise for his unparalleled vision, dribbling and wand of a left foot among other incredible attributes.
But not too much is made about Messi's mentality and desire to win, something Koeman has pointed out in a new interview.
Having worked with him during his final season at Barcelona, the Dutchman found Messi to be laser-focused in training and utterly perfect in the rondo exercises.
When his team lost a rare training game, he was absolutely furious.
Speaking to Voetbal International, via Goal, Koeman said: "When we did a finishing practice during training, there were sometimes players who started to hit easy balls, a bit of fooling around. But with Messi everything was: boom, boom, boom, boom.
“Never frills, everything functional. And always wanted to win everything.
“We always play a rondo before training. If the ball goes around 20 times, then the players in the middle must have an extra turn. If that happens three times in a row, the players will form two lines and the two who were in the middle then walk through and get taps on their heads and such.
“I asked Messi if it had happened to him once. ‘Yes, once’, he said. In all those years.
“With him, the older players never lost an exercise against the young. It happened once and Messi was seriously angry about that for a week. Really, a tyrant.”
When Koeman took the reins at his old club, Messi was desperate to leave Barcelona and submitted a transfer request.
However, he ended up staying put and hit 38 goals in all competitions in his last season for the Blaugrana - before departing for Paris Saint-Germain when La Liga wouldn't accept his new contract.
He apologised to Koeman, who waxed lyrical about Messi and believes he helped hide how dire a situation Barca were in with his greatness.
Koeman continued: "Lionel Messi covered all the flaws at this club. He is so good. Of course he still had some good players around him, but he was the one who made the difference.
"And he made everyone around him look better than they are. This is not me criticizing, this is my observation. Messi is a 10/10 in every aspect of the game. In recognizing situations on the pitch, in finishing, in controlling the ball, in putting the right speed behind a pass.
"It's not normal, it's not normal! When I just became manager here, I went to Messi's house. He was extremely concerned and interested.
"The start of last season was difficult and he apologized to me later on for that. Messi needs to have an idea that there's a future within the club."
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