The Dutch manager has overseen four La Liga matches since taking over in the summer - two wins, one draw, and one loss - in which Messi has only scored a penalty in a 4-0 victory over Villarreal. The 33-year-old mustered one shot on target in Saturday's 1-0 defeat at Getafe.
"Maybe, at the moment, Messi's performances could be better," Koeman said on Monday, as reported by ESPN's Sam Marsden.
"But he is happy, he is working hard and he wants to be the captain. I don't have a single complaint. He has also had some bad luck. He hit the post (on Saturday) and on another day that goes in.
"I don't have any doubts about his performances and we're going to see him (at his best) in the upcoming games, I'm sure."
Messi's below-par outings follow the whirlwind 10 days that followed his formal request to leave Barcelona in August. He eventually decided to stay rather than become embroiled in a legal battle with the club's hierarchy. However, when he withdrew his request he took the opportunity to say he's unhappy at Camp Nou and accused embattled president Josep Maria Bartomeu of breaking the promise that would allow him to exit.
Barcelona begin their Champions League bid against Hungarian champions Ferencvaros on Tuesday and Koeman has been quick to talk down his team's chances of success in the tournament.
"I can't explain what happened in recent seasons because I wasn't here - there were different players and coaches - but if we fail this season, I will be able to tell you why," he said.
Real Madrid visit Barcelona for the latest edition of El Clasico on Saturday. Zinedine Zidane's Real Madrid have also begun the season in unconvincing fashion and were beaten 1-0 by newly promoted Cadiz over the weekend.