With left back Jordi Alba sidelined and captain Carles Puyol yet to return from a knee surgery, Mascherano's injury is a fresh blow to coach Gerardo Martino ahead of Tuesday's Champions League Group H match at Celtic.
The Argentina international sustained the injury in a collision with team mate Sergio Busquets in the second minute at the Nou Camp and eventually had to be replaced by Marc Bartra just after the half hour.
"A strain in the right thigh muscle was confirmed which will put him out of action for 10-15 days," Barca said on their website on Wednesday.
Mascherano's absence may give 22-year-old youth academy product Bartra a chance to shine, although Puyol is close to a return to competitive action.
As well as the Celtic match, Mascherano is likely to miss Barca's La Liga games against Almeria and Real Valladolid.
He may recover in time to feature for his country in their World Cup qualifiers against Peru on October 11 and Uruguay on October 15, although Argentina are already through to next year's finals in Brazil.