The forward is out of contract at the end of June, when he will also turn 34, and he has not yet agreed to a new deal with the club. Next week's match at Eibar, therefore, could be his last in a Barca shirt.
"I hope not because I think [against Celta] he's shown his effectiveness and that he's still the best in the world," Koeman said in a news conference when asked if Messi had played at Camp Nou for the final time.
"It [would be] impossible to play without him. But it's Leo on his own and, to win more games and get more points, we need more players to score goals.
"Leo's scored 30 goals and won us a lot of points. 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."
ESPN reported earlier this year that Barca president Joan Laporta is working on a two-year deal with the option for an additional season which he hopes will convince Messi to remain at the club he first joined 20 years ago.
"I hope not, but it's his decision," Jordi Alba told Movistar when also asked if Messi has played his last game at Camp Nou. "I hope it's not his last."
Messi reached 30 league goals for the ninth time in his career with a header against Celta but a brace from Santi Mina stretched Barca's winless run to three games.
Just one win in their last five games has seen the Blaugrana throw away the chance of winning the league and has increased the speculation surrounding Koeman's future at the club.
Even prior to the midweek draw with Levante and Sunday's defeat against Celta, sources told ESPN there were "growing doubts" among the board regarding the Dutchman.
"That's not a question for me," Koeman said when asked if he would be in charge next season.
Koeman met with Laporta last week and said on Saturday that he hopes to continue beyond the summer and that he's arranged to meet with the president again upon the conclusion of the season.
Results elsewhere on Sunday mean Barca are guaranteed to end the campaign in third place, regardless of what happens next weekend.
Koeman said the season, which saw Barca win the Copa del Rey but knocked out of the Champions League by PSG in the last 16, should be analysed in full, "not just on results over the last two weeks."
With Barca out of the picture, Atletico Madrid and Real Madrid will compete for the title on the final day. If Atletico win at Real Valladolid, they will be champions, but a draw or a defeat would open the door for Madrid, who host Villarreal.