Madrid - Lionel Messi is undergoing treatment on kidney stones which floored the Argentina superstar at the Club World Cup in December, Barcelona announced on Monday.
The five-time world Player of the Year "will be submitted to various tests on Monday and Tuesday to assess the development of kidney problems he suffered last December", the Spanish champions said in a statement.
Messi is due back on Wednesday, when Barca face Valencia in the Copa del Rey semi-final second leg, with the Catalans firmly in control after the 7-0 first leg drubbing they handed Gary Neville's team as Messi bagged a hat-trick.
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The five time World Player of the Year is reported to be receiving lithotripsy, a procedure involving ultrasound shock waves to break down problem kidney stones.
"The surgery normally lasts between three quarters of an hour and an hour with the patient requiring sedation or in some cases anaesthesia," the Mundo Deportivo newspaper reported.
Messi suffered renal colic, a problem commonly caused by kidney stones, in Japan in December forcing him to miss eventual winners Barcelona's semi-final against China's Guangzhou Evergrande.
He recovered in time to line up in the final where Barca beat Argentina's River Plate 3-0 as he scored the opening goal.