Messi and Barcelona have parted company after over 20 years together, but Koeman has made it clear that there's no time to dwell on the loss of their captain, and talisman.
"It's normal that when a player like Messi leaves, it's hard for everyone, but we have to move on because we can't change the situation," Koeman said after their Joan Gamper Trophy defeat of Juventus.
"We have to play well, win and work even harder. There are other players who can play in his position and play an important role in the team.
"We don't have Leo now, though. We have to move on and look forward."
The Dutchman did give Barcelona fans reason to be optimistic heading into next season.
"We've signed Memphis Depay, who has shown today that he can be really effective," Koeman explained.
"The midfielders need to score more and the squad needs to improve. The best player in the world will always be missed, but you have to accept it and get to work.
"It's no use dwelling on the past. There's a lot of promise in this team."
Koeman was happy with the performance of his Barcelona side ahead of the start of the new campaign.
"It's a really good result. Football wise, we reached a really good level," Koeman said.
"Neto was very good and we've taken another step forward. The boys who were at the Olympics still have to return, so when we have everyone back, it'll be better.
"We're on the right track to competing well in the first match."
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