BARCELONA, Aug. 5 (Xinhua) -- Argentine star Lionel Messi strained his right calf in his first training session on Monday, forcing him to extend his break and miss the upcoming matches against Serie A side Napoli.

"Messi withdrew from the session due to discomfort in his right leg," Barcelona said in a club statement. Barca confirmed that Messi was diagnosed with a grade 1 calf strain. The club did not say when Messi could return to the field.

Messi also expressed his disappointment on social media: "I wanted to be with the team and the fans that follow us in the U.S. This time its not to be but we will see each other soon," he wrote.

Barcelona will play on Thursday at Miami Gardens, Florida, and on Saturday in Ann Arbor, Michigan. Barcelona faces Napoli on both Aug. 7th and 10th, before opening their league schedule on the 17th against Athletic Bilbao.