The Argentina international was stretchered off in the closing stages of Barcelona's 2-2 draw with Paris Saint-Germain on Tuesday, with the club confirming a ligament tear.
Mascherano was stretchered off in the closing stages of the Champions League quarterfinal first leg clash at Parc des Princes, with Marc Bartra coming on to replace him.
The club confirmed on its official website that the Argentina international has suffered a ligament tear in his right knee, ruling him out of action until the closing weeks of the season. Mascherano had picked up a yellow card earlier in the second half on Tuesday evening, meaning he would have been suspended from the clash at Camp Nou next week regardless of his injury.
The news comes as a bitter blow to Barca as its injury problems continue to mount. Lionel Messi was withdrawn at halftime against PSG with a hamstring problem, while captain Carles Puyol is unable to play again this season after undergoing knee surgery last month.
A last-gasp Blaise Matuidi goal secured the draw in Tuesday's game, after Xavi's late penalty and Messi's first-half strike had seemingly secured a win for the Blaugrana despite a goal from Zlatan Ibrahimovic.