The 27-year-old has returned to Juve for a second spell after spending half a season on loan at the Turin club last year, signing a five-year contract in a deal worth 38 million euros ($44.35 million) earlier this month.
Juve have signed the former Paris Saint-Germain forward as a replacement for Dusan Vlahovic following the Serbian's departure to Besiktas.
In his first spell at Juve, Kolo Muani scored 10 goals in 22 appearances before PSG loaned to Tottenham Hotspur for the 2025-26 season.
Spalletti told broadcaster DAZN that Kolo Muani should have been on the scoresheet in their Serie A opener on Sunday. Brazilian centre-back Gleison Bremer netted the only goal of the game.
"We went out and signed him (Kolo Muani), we've looked after him and we know his worth, but he needs to realise he hasn't played for two years and there must be a reason for that," he said.
"Sometimes you think you're at an exceptional level, but then you've left something behind and you have to go and get it back.
"Kolo knows how to hold onto the ball, find space and is physically strong, but these are the sort of matches where he needs to score goals."
Juve next host Parma on Saturday.