Lionel Messi was like a "tyrant" in training and helped to "disguise everything" wrong at Barcelona in recent years, according to head coach Ronald Koeman.

Barca were left without their talisman in August when Messi departed for Paris Saint-Germain on a free transfer after they were unable to fulfill a new contract that had been agreed with the superstar forward.

The six-time Ballon d'Or winner scored 672 goals and assisted 265 more across 778 appearances during a trophy-laden 17-season spell at Camp Nou.

Koeman's side have made a slow start to the post-Messi era, having dropped points to Athletic Bilbao and Granada in their first four LaLiga games and lost 3-0 to Bayern Munich in the Champions League.

And with his job reportedly under threat amid speculation Barca are ready to move for Roberto Martinez, Koeman accepts Messi's performances papered over the cracks for too long.

"Lionel Messi disguised everything. He was so good and he won," Koeman told Voetbal International. "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. I knew how good he is, but it's still nice to see it up close every day.

"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!

"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 he 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 failed in an attempt to force through a move away from Barcelona last year and went on to score 38 goals in 47 games last season – 18 goals more than Barca's next-highest scorer Antoine Griezmann, who has also departed the club.

That includes 30 goals in LaLiga, which was an increase on the 25 managed in the season before Koeman arrived.

Koeman fears Messi's departure could have a major knock-on effect, with the standards of some players – fellow academy graduate Ansu Fati among them – slipping since Messi signed for PSG.

"He delivered 50 goals last year: 30 goals, 20 assists. He was an example for others," Koeman said. "When we won the cup, Messi had won so many trophies, bigger than this one, but you saw that the Copa del Rey really did something for him this time. 

"With those young players in there, he saw the future of the club. All those guys wanted to take a picture with him, I've never seen anything like it. That's how big he was and is. It was a shock to the whole town that he's not here now."

Koeman added: "The performance with Messi is perfect. We now sometimes do a pass and kick practices, where you had to shoot the ball into a small goal, around a post. 

"The other day I saw Ansu Fati shoot three metres wide, simply due to lack of concentration. But he wouldn't have done that if Messi had still been there. Then, he got set right, Messi became furious. That never happened to [Messi].

"I have never met anyone except Johan Cruyff with his football intelligence. Alfred [Schreuder, Koeman's assistant] sometimes explained the exercises in English and Messi does not speak that very well. But after a few seconds he knew it."

Despite his job being under threat ahead of Thursday's trip to Cadiz, Koeman stands by his decision to step down as Netherlands boss in August 2020 to take over at Camp Nou.

"I do not regret signing for Barcelona and leaving the Dutch team," he said. "It's in situations like these that I get the best out of myself and my team."