Messi, 33, has played every minute of Barca's nine games in all competitions since Koeman was appointed but has yet to score a goal from open play. He has scored four penalties.
"I have said many times now, Messi still has a lot of quality and he's important for our attack," Koeman told a news conference when asked if the Argentina international was in decline.
"I know he's [only] scored penalties but we're used to him making the difference and he's still a decisive player. We want him to be on the pitch where he can do damage to the opposition.
"He always makes the difference and he's very important to our way of thinking, above all in attack -- whether as a false 9 or as a winger coming inside."
Messi's performances at Barca are in the spotlight more than ever at the moment, not just because he is moving into the final years of his career but because he tried to leave the club in the summer.
Asked how about Messi's mood, Koeman replied: "He's in good spirits, strong. He always enjoys this sport. He's a winner, as he shows every day.
"Like any player, he has difficult moments or times when the ball doesn't go in for whatever reason. But from what I have seen he's been very involved."
Messi even drew criticism this week for failing to press in the final minutes of Barca's 2-1 Champions League win against Dynamo Kiev on Wednesday.
"If you want to make problems, that's up to you [the media]," Koeman said when asked about the images. "I don't agree with trying to stir up controversy.
"You can take images of any player when they're walking. I have not seen it and it doesn't interest me. I care about his attitude and it's been very good."
Of Messi's four penalties this season, only one has come in La Liga and he's without a goal in his last five league games. Barca, meanwhile, have picked up just two points from their last four top-flight matches.
Messi will look to end that mini drought on Saturday when Barca welcome Real Betis to Camp Nou in search of their first league win since Oct. 1 following their worst start to a season in 18 years.
"Teams like Barca must be up the top of the table," Koeman added. "If not, it is because we are dropping points and losing games.
"I accept the criticism that comes with that but we're not too obsessed [by the table] because it's a long season."