"Tests carried out tonight have confirmed that the player has a bruising of the bone on the outside of the left knee," Barca said in a statement.
"The rate of his recovery will determine whether he will be available for the next game, on Sunday against Real Betis."
The World Player of the Year was carried off on a stretcher near the end of their Champions League Group G match at home to Benfica.
The Argentine collapsed to the ground clutching his knee after colliding with Benfica goalkeeper Artur and looked to be in severe discomfort as he was taken from the pitch.
The initial diagnosis was that he had suffered only bruising.
"After beating the offside trap Messi tried to dribble round opposing keeper Artur, who dealt him a blow, but Messi continued with the move," Barca said in a statement on their website.
"After shooting with his left foot the player fell to the ground with pain in his left knee," the statement added.
The game ended 0-0 with Barca already through to the last 16 as group winners and Benfica forced to settle for a place in the Europa League.
Messi remains one goal short of Gerd Mueller's record of 85 goals in a calendar year set in 1972.
The last time Messi suffered an injury was in September 2010 at Atletico Madrid when a hefty challenge left him with ankle damage that ruled him out of two games, the club said.
