"Leo has seen for quite some time that the team is improving thanks to all the changes we've made," Koeman said after Barcelona bowed out of the Champions League.
"Particularly, we have young players of great quality. We've got a great future ahead. Leo can't have any doubts about what the future holds for this team."
Despite being eliminated in the round of 16 - the earliest Barcelona have stumbled since the 2006-07 season - the club is optimistic Messi will sign a new contract before his current deal expires on June 30. Newly elected club president Joan Laporta said he's convinced the Argentine wants to stay.
Messi will decide on his future at the end of the campaign.
Koeman has breathed new life into the club since taking over last summer, earning praise in particular for incorporating more youth players into the first team. Academy graduates Pedri and Oscar Mingueza both started Wednesday's second leg against Paris Saint-Germain, adding to a young core featuring the likes of Ansu Fati, Ronald Araujo, and Riqui Puig.
And after an indifferent start to the season, the Dutchman has steered Barcelona to the Copa del Rey final and back into the La Liga title race. The Blaugrana are now just six points behind first-placed Atletico Madrid with 12 matches to go.
Though Barcelona lost 5-2 on aggregate to PSG, the Spanish giants looked fresher and sharper in Wednesday's second leg. Koeman said they "deserved" a better fate after the 1-1 draw at the Parc des Princes.
"In the first leg, (PSG) were ultra effective in front of goal, and in this match, we weren't," the 57-year-old added. "They scored four, here we scored once. That comparison of efficacy is the big difference across this tie. But no criticism of the lack of cutting edge. We put in a huge effort and merited more, maybe just a hint of luck. Their keeper was (the) player of the match."