Ansu Fati is filling the void as Barcelona's new number 10

"He can't be expected to fill the gap Messi left behind, it's impossible," said Ronald Koeman, answering the question nobody had asked but everyone was thinking.

Ansu Fati scores goals, wears number 10 and is by far the most exciting thing happening at Barcelona, a club mourning the loss of their best ever player and recovering from financial meltdown.

In Sunday's Clasico at Camp Nou, still the most-watched club fixture around the world, the 18-year-old will be Barcelona's greatest hope of victory.

Koeman knows it, after resisting the temptation to play him from the start against Dynamo Kiev on Wednesday, despite Barca realistically needing to win to avoid being knocked out in the Champions League group stage.

Even on the bench, Fati was influential, his warming-up on the side enough to send a jolt of electricity around Camp Nou, with Gerard Pique scoring the only goal of the game minutes later.

After 11 months out with a knee injury, Fati is being preserved by Koeman like the precious jewel he is, held back even as the club is desperate to see him run.

Fati runs fast and slow, like Messi, a master of the change of pace. He can dart between opponents or surge round them. He shoots quickly, catching goalkeepers and defenders off guard before they have a chance to get set.

"He has a surprising maturity, the ability to mix moments of explosiveness with moments of calm," said former Real Madrid player and manager Jorge Valdano. "When I see this sort of thing in a young player, I take them very seriously."

Before Messi departed for Paris Saint-Germain, he promoted Fati's rise. On the pitch, they each thrived on the other's feathered touches and quickness of thought, while off the pitch, Messi's brother Rodrigo was Fati's agent before Fati hired Jorge Mendes.

In August 2019, after Fati made his debut aged 16, Messi posted a photo of them sharing an embrace and it received more than six million likes on Instagram.

"I was impressed the moment I saw him," Messi said.

The idea of a succession was almost cultivated by Messi and if it was helpful to Fati then, it may not be now. The expectation of what he might be is possibly overtaking the enjoyment of what he is.

Yet there is also a recognition the void Messi left does have to be filled, not in terms of performance, which as Koeman says is "impossible", but a reference point, a talisman and symbol of hope.

And if somebody has to wear number 10, who better than Ansu?

"Ansu Fati is the cornerstone on which to rebuild Barca," wrote Lluis Mascaro in Catalan newspaper Diario Sport on Friday. "Messi is history. The best and most wonderful story ever written. But history. And now it's time to bet on the future."

If the pressure is enormous, there is no sign yet of it being a burden.

Fati's quiet, unassuming persona even chimes with the way Messi handled stardom, that approach admired perhaps in part because it became such an obvious counter-point to Cristiano Ronaldo.

When Fati was growing up in Guinea-Bissau, a small country in West Africa ranked among the poorest in the world, he played on red sand, wearing sandals. "He still dribbled round players twice his size," Malam Romisio, his mentor at the time, told AFP.

After moving to Spain to join his father Bori, who made ends meet by picking olives, cleaning glasses in nightclubs and driving trucks, they found a base in Herrera and Fati spent two years with Sevilla before joining Barcelona's famed La Masia.

"After one of his first tournaments, he took the train back to Seville on his own and sent me a message," his first Barca coach, Marc Serra, told AFP.

"He wrote he was very sorry for playing so badly and it would not happen again. He was ten, but already he was mature and determined."

Before taking on Messi's number 10 shirt, Fati asked permission from the club captains - Pique, Sergio Busquets, Sergi Roberto and Jordi Alba. "Nobody will equal what Leo has done, I have my path to follow," Fati said on Wednesday, after signing a new six-year contract.

That commitment was particularly welcomed, because of his status and talent but also because he committed to the club at a time many are leaving or reluctant to join. The credit is likely to last.

"There were offers from elsewhere but I was clear," said Fati. "My dream has always been to succeed at Barca."