Koeman took over at the Catalan club last summer and worked with Messi a year before the Argentine left the club for Paris Saint-Germain.
While the Dutchman was well aware of the six-time Ballon D’Or winner’s ability, his intensity in training amazed Koeman after the former Southampton and Everton manager was appointed at the Nou Camp.
“I knew how good he is, but it’s still nice to see it up close every day,” Koeman explained to Voetbal International.
“Everything you would like to teach a football player, in recognising situations, in taking the ball under pressure, in ball speed, in finishing; with Messi everything is a 10. Not normal, not normal!
“Of course he had good players around him, but he made the difference. Everyone seems better than they are because of him. This is not a criticism, but an observation.
“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.”
Messi left Barcelona for PSG this summer after the club fell into financial difficulties and could not keep the Argentine despite reaching agreement over a new deal. It marked the end of a 20-year relationship between Messi and the club since his family relocated to Catalonia in 2001.
The forward is yet to get off the mark in a PSG shirt and suffered a knee injury during Mauricio Pochettino’s side’s win over Lyon on Sunday.
Barcelona, meanwhile, are unbeaten in La Liga but were well beaten by Bayern Munich in their opening Champions League group game.
They also required a last-minute equaliser from Ronald Araujo to snatch a point off of visitors Granada on Monday.
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