Lionel Messi has been ruled out of Barcelona's squad for the two games against Napoli in the United States this week after injuring his calf in his first training session back.

Messi picked up the knock during training on Monday and will not travel to the U.S. with the rest of his teammates, including new signings Frenkie de Jong, Antoine Griezmann and Junior Firpo.

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"Messi withdrew from the session due to discomfort in his right leg," Barca said in a statement.

"Tests carried out have diagnosed a grade one calf strain for the Argentine. Messi will remain in Barcelona and will now not travel to the USA for the club's tour. His availability to come back to action will be dictated by his recovery from the injury."

The Argentina international's participation in the Copa America meant he wasn't cited to return to training until Monday. However, while he didn't play, he was present for Sunday's win over Arsenal at Camp Nou.

Messi expressed his disappointment on Instagram, posting a message that read: "I was looking forward to getting started but unfortunately I have had a setback in my first training session that's going to rule me out for a short while.

"I wanted to be with the team and the fans that follow us in the U.S. This time it's not to be but we will see each other soon."

Speaking in his role as captain before the game, he told supporters he didn't regret pledging to win the Champions League last season despite the humiliating loss against Liverpool in the semifinal.

"It's difficult to say anything after last season," he said. "But I don't regret anything, and I would say the same thing again. I back these players and the coaching staff we have. Together we're going to fight to win everything.

"Last season ended with a bitter taste, but we have to give value to La Liga, the eighth we have won in 11 years. That's important, and maybe we don't give enough value to what we have done."

Neither Messi nor Luis Suarez were part of the club's tour to the U.S. last summer because of their involvement at the World Cup.

Of the other four South American players who saw action in Copa America, Suarez is confirmed to travel, along with Arturo Vidal, Philippe Coutinho and Arthur Melo.

B team players Inaki Pena, Riqui Puig, Carles Perez and Abel Ruiz have also been included.

Barca play Napoli at the Hard Rock Stadium in Miami on Wednesday before flying to Michigan to face the Italians again as they step up preparations for their curtain-raiser against Athletic Bilbao next weekend.

The Spanish champions are also holding a number of fan events while in the U.S. as they look to get closer to their American supporters -- a market of particular interest to president Josep Maria Bartomeu.

Some of the first-team squad are due to appear at a "Barca Fest," which is being held at Bayfront Park in Miami on Tuesday, with artists Pedro Capo and Play N Skillz set to perform.