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Fati, 17, was in devastating form for Barca at Camp Nou as Villarreal was swept aside, scoring two fine first-half goals in a five-minute period, before then winning a penalty that was converted by Lionel Messi.

With Messi's future seemingly unclear, Fati had been billed as the poster boy of a new era at Barca – fittingly it was the Spain international who got the ball rolling in style.

While Koeman was impressed – the Dutchman had been shown beaming as he embraced Fati after his 70th-minute withdrawal – he was keen to remind both the player and fans that the youngster still has much to learn.

"We must highlight the game and performance of Ansu Fati," Koeman said. "I was complaining a little last week about him not having the maximum level of performance in the pre-season.

"He is only 17 years old, he will have a great future, but he must improve things. Sometimes he lacks a bit of concentration.

"One of my criticisms was in the friendly against Elche – he was not good. Today he was. His intensity out wide was immense, he played a great game and we took advantage of his qualities."

Sergio Busquets hit a similar note, stressing the need for patience with Fati.

"We have to be calm with him, we know the potential he has," he said. "He is tremendous for his age, but we don't want to get too far ahead of ourselves.

"He has many team-mates from whom he can continue to learn so that his ceiling is as high as possible.

"You have to let him play his football and take advantage of the fact that he is at a good point."