Ronald Koeman says Ansu Fati still has plenty of room for improvement and does not expect the youngster to achieve the "impossible" by completely filling Lionel Messi's void.
The 18-year-old has committed his long-term future to Barca by agreeing terms on a new contract, which Los Cules announced on Wednesday.
Fati inherited Messi's number 10 shirt following the latter's shock switch to Paris Saint-Germain and is considered Barca's big hope now the club's all-time leading scorer is no longer around.
But he was guilty of missing a great opportunity in Wednesday's Champions League clash with Dynamo Kiev, which Barca only won 1-0 despite controlling the contest.
The Spain international has scored 15 goals in 48 games for Barca in total, a tally that is bettered only by Luis Suarez (21), Antoine Griezmann (35) and Messi (69) – none of whom are still at the club – since his debut 26 months ago.
Still regaining fitness after nearly a year on the sidelines with a knee injury, Koeman does not want supporters to get carried away when it comes to Fati's development.
"Ansu's chance today shows he still lacks things. He needs to improve," Koeman said at his post-match news conference.
"That is entirely normal. He will only turn 19 at the end of the month. He cannot be expected to solve all our problems, or what Leo Messi left behind. It's impossible.
"You have to help each other go step by step. We cannot expect miracles, especially when he has been out injured for a long time and is still only 18."
Barca had 11 shots against Dynamo but only found the target from three of those, with Gerard Pique scoring what proved to be the winner with 36 minutes played.
As well as becoming Barca's oldest Champions League goalscorer at 34 years and 260 days, Pique is now also the joint-leading goalscorer among defenders in the competition's history, level with Roberto Carlos (16 each).
Koeman felt his side should have won by a more comfortable margin against accommodating opponents Kiev, who failed to muster a single attempt on target at Camp Nou.
"I'm happy with the performance, but not because of our effectiveness," he said. "In a game when we were so superior and created opportunities, we should have scored more.
"If they don't have opportunities, you can't ask for more from the attackers, but they did. The normal outcome today would have been 3-0 or 4-0.
"I understand that people leave upset, because I am too. We have achieved the result we wanted, but with the opportunities we have created, we have to score more."
Barcelona's victory was their first in the Champions League in six attempts since December 2020, a run that includes back-to-back 3-0 losses to Bayern Munich and Benfica to begin their 2021-22 campaign.
The LaLiga club are now back in contention for a top-two finish in Group E at the midway point, but full-back Jordi Alba – who set up Pique's winner – accepts his side were made to work harder for the points than they perhaps needed to.
"In the first half we had chances to score more than once," he told Movistar. "In the second half we weren't so good under pressure and we suffered a bit.
"They did not create opportunities but we were still not in a good place. We have to improve on the second-half display."
In a worrying update ahead of Sunday's Clasico clash with Real Madrid, Alba confirmed he sustained an injury during the Kiev match despite playing the full 90 minutes.
"It's my toe. We'll have to wait and see what it is," he said. "But now we look forward to the Clasico. We have won our last two games and the fans have responded to that, helping us grow.
"We are expecting a tough game against Madrid, but we hope to bring more joy to the fans."