The 18-year-old signed a new long-term contract with the Camp Nou outfit on Wednesday following his recent return from injury.
Such is the talent at his disposal that Fati has been entrusted with the famous number 10 jersey at Barcelona following Lionel Messi's exit to Paris Saint Germain in the summer.
But Fati was reminded of how difficult it will be to fill the boots of the six-time Ballon d'or winner when he missed a good chance against Dynamo Kiev in Barcelona's 1-0 win over the Ukrainians in the UEFA Champions League on Wednesday.
Fati has already scored twice this season since his return from injury but Koeman says it will be impossible for the Spain international to replicate Messi's heroics for the club.
"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."
Messi made 778 appearances for Barcelona after making his debut in 2003 and is the club’s all-time top scorer with 672 goals. He won 35 trophies with the club including four UEFA Champions League trophies and 10 La Liga titles.