Barcelona, Spain: Lionel Messi's availability for Barcelona's Champions League opener with Celtic next week remains in doubt after tests confirmed the five-time World Player of the Year is suffering from a groin injury.

Messi returned to Spain after aggravating the injury as he scored the winner against Uruguay last Thursday in his first match with Argentina since reversing his decision to retire from international football.

In a statement on Monday Barcelona said tests had confirmed an injury to his left groin and that Messi will "lower the demands of his training sessions in the next few days."

The Spanish champions refused to state how long Messi will be sidelined for, but Barca are unlikely to take any risks for Alaves' trip to the Camp Nou on Saturday with seven matches to come in 22 days.

"Everything is fine, I have to slow things down as I recover with physio, but it's nothing to worry about," Messi posted on his official Facebook page.

Messi had started the season in fine form, scoring three times as Barca won the Spanish Super Cup against Sevilla and opened their La Liga title defence with back-to-back victories over Real Betis and Athletic Bilbao.

Argentina boss Edgardo Bauza said Messi had gone against the wishes of Barcelona's doctors to make a triumphant return to the national team having dramatically reversed his shock decision to retire from international duty after missing a penalty in Argentina's Copa America final defeat to Chile in June.

Messi will miss Argentina's latest World Cup qualifier away to Venezuela on Tuesday.