Bartomeu said he had seen a change in the Argentina international, 31, who was named Barca skipper in August after Andres Iniesta’s departure from the Camp Nou.
“Messi is very happy here. He is delighted and excited about continuing to triumph at Barcelona,” he told Onda Cero on Monday.
“Messi has made a change, he has decided to take on the captaincy and assume that responsibility in the team.”
Barca have come in for heavy criticism after they agreed to play a La Liga game against Girona early next year in the United States.
Bartomeu said his club were simply trying to grow the league.
“La Liga proposed that we play a match abroad and it seemed a good idea. We have the obligation to promote the league,” he said.
“You have to get closer to the fans and find new streams of income. Playing a match abroad would help us. If it can be played, we’ll do it. The club captains have been informed.”
Philippe Coutinho is desperate to make up for lost time as Barcelona prepare to launch their latest bid for Champions League glory.
The 26-year-old Brazil international made his £142million move from Liverpool to the Nou Camp in January having already played for the Premier League club in the competition and was a frustrated by-stander as they went out to Roma in the quarter-finals.
He could finally get his chance when Barca entertain Dutch champions PSV Eindhoven as Group B gets under way on Tuesday evening, and he admits he cannot wait.
Coutinho told a press conference: “It will be a special, important night for me. I’m looking forward to this because I couldn’t play in this competition for Barca last season.
“We are Barca and we want to win every competition we play in. The Champions League is a huge competition and we want to do big things.
“It’s a very demanding competition because of the quality of the teams involved, but both us players and the fans are really hungry to win it.”
Coutinho has been compared favourably to now-departed Barcelona great Andres Iniesta, and while the Brazilian is flattered to be mentioned in the same breath as a man he regards as a “genius”, he insists he has to play his own game.
He said: “It’s hard to compare me to him because we both have our ways of playing. I just try to be myself – not to say that it isn’t a huge honour to be compared to a player like him.”
If Iniesta has gone, Lionel Messi is very much still at the forefront of Barca’s quest for a sixth Champions League success, but Coutinho knows he and his team-mates cannot afford to get ahead of themselves.
He said: “He (Messi) is the best player in the world and surely of all time. Like the rest of us, he wants to achieve great things this season and the Champions League is one of those.
“We are in a very difficult group. We need to be 100 per cent in every game and not think that we are favourites.”
The men from Catalonia will head into the game on the back of a perfect five-game start to the season, although PSV have won seven on the trot since their penalty shoot-out defeat by Feyenoord in the Dutch Super Cup on August 4.
The visitors are managed by former midfielder Mark van Bommel, who won the competition with Barcelona in 2005-06, and he will return refusing to be bowed by the task ahead of his team.
He told a press conference: “There are people who say that if you go to Barcelona and you only lose 1-0, 2-0 or 2-1, you did a good job.
“But that’s not our goal and if you’re going to think like that you’d be better off staying home. Once or twice a season, there’s a surprise.”
Iniesta was the latest club legend to depart, leaving this summer for Vissel Kobe in Japan, following the lead of Xavi, who left for Qatar in 2015.
“Andrés and Xavi are unique,” Messi told the Barcelona website. “They made the game more colorful and brought more possession.”
When asked which current teammate he has the best connection with on the pitch, Messi responded, “Jordi Alba knows me to perfection.
“He knows when I will give him a no-look pass. I have a special connection with him.”
Like Messi, Alba was part of Barcelona’s youth setup, though he left the club to establish himself, at Valencia, before returning in 2012.
Messi also praised teammate Luis Suarez, one of his best friends at the club, hailing the impact the Uruguayan striker has on the team.
“He brings a lot to me and to the rest of the team,” explained Messi.
“He leads us in creating pressure and provides strength to all of us.”