Messi has renal colic, a type of abdominal pain commonly attributed to kidney stones, the club says.
Neymar is in the reserves.If Barcelona reach the final on Sunday, Messi's availability will "depend on the outcome of further medical tests," the club adds.
The news is a major blow to Barcelona, which has not won its three previous matches entering the semifinal.
The winner will face South American champion River Plate in the final.
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