Inter Miami star Lionel Messi was suffering from muscle fatigue in his left hamstring when he requested a sub on Sunday during a 6-4 win over the visiting Philadelphia Union, the club has announced on Monday.
"After undergoing further medical tests this Monday, the initial diagnosis indicates an overload associated with muscle fatigue in his left hamstring," Inter Miami said in a statement. "The timeline for his return to physical activity will depend on his clinical and functional progress."
Messi was seen grabbing at his upper thigh before he asked to sub out in the 73rd minute, and he headed directly to the locker room.
Messi's ailment comes about one week before Argentina begins to prepare for its World Cup title defense at a training camp in Kansas City, Kan.
It's unclear whether Messi will have to miss any time due to the overload. The Major League Soccer season has now begun its World Cup break, and Messi, 38, may return to training with Argentina or may need to rest further before matches begin.
Argentina plays Algeria on June 16 in Kansas City, Mo., for their first group-stage match. Messi and company will also face Austria and Jordan as part of Group J.
Messi helped Argentina win its first World Cup since 1986 and its third overall four years ago in Qatar. He won the Golden Ball for the tournament's top player.

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