The Argentina international failed in his attempt to engineer a move away from Camp Nou during the close-season but is set to become a free agent at the end of the campaign.
He has not always been at his best this season and has admitted in a new interview with LaSexta that the saga surrounding his future has affected his performances.
"The truth is today I'm fine," Messi said. "Today I'm fine and I feel like fighting seriously for everything we have ahead of us.
"I'm excited. I know the club is going through a difficult time, at club and team level, and it is difficult everything that surrounds Barcelona but I'm looking forward to it."
Messi has gradually improved as 2020-21 has gone on and has five goals in his last seven LaLiga matches, having failed to score or assist in five matches in October.
The superstar forward's comments were welcomed by Koeman but the Dutchman insists he never doubted Messi's determination to succeed after seeing his exit blocked.
"I don't need to see the Messi interview to know just how hungry and motivated he still is," Koeman said at a news conference previewing Tuesday's LaLiga trip to Real Valladolid.
"I see him with his team-mates every day. It's tough for any player who has won as much as him to not win every match he plays.
"But the squad is very united, albeit unhappy with the results. Leo is integrated in the side and helping us to improve things."
Messi's goal in Saturday's 2-2 home draw with Valencia was his 643rd for Barcelona, equalling Santos great Pele's one-club goals record.
Barca is eight points adrift of Atletico Madrid having played a game more than the leader and is winless in four away league matches, last going five without a win in 2008.
Koeman accepts that results have to improve, with the under-fire head coach putting his side's inconsistent results down to a high turnover of youngsters in the first team.
"We have changed a lot of things," he said. "With younger players come a bit of irregularity. For the future it is a good thing but at Barca we have to win titles.
"It is a complicated situation we are in and time is needed, but we are Barcelona and we want to win.
"We will need to have maximum concentration against Valladolid. Every game we do not win is a step backwards for us.
"There are many games ahead in which many things can happen. We have lost a few away games and cannot fail again. There is pressure on the team to close the gap."
Ousmane Dembele is back in full training following another spell on the sidelines with a hamstring issue aggravated earlier this month.
However, Koeman confirmed the France international will play no part at the Estadio Jose Zorrilla, while Gerard Pique, Ansu Fati and Sergi Roberto are also out.
Barca has won 12 of its last 13 LaLiga games against Valladolid, its only defeat in that run coming in March 2014.