Messi headed in his 30th league goal of the season, as Barca lost 2-1 in their final home fixture of the year.
The result saw their title hopes end mathematically.
Messi is out of contract at the end of June and he has not yet agreed to a new contract with the club.
Next week’s game at Eibar, therefore, could be his last in a Barca shirt.
“I hope not because I think [against Celta] he has shown his effectiveness and that he’s still the best in the world.
“It [would be] impossible to play without him. But it is Leo on his own and, to win more games and get more points, we need more players to score goals.
“But [his future] is a question for him. For me and for the club, we hope he stays. Because if there’s no Leo, we have doubts about who will score the goals,” Koeman said in a news conference when asked if Messi had played at Camp Nou for the final time.
