The Catalans were already short-staffed in central defence due to the prolonged absence of France international Samuel Umtiti, who is out with a knee problem and with no date set for his return.
"Medical tests carried out this Monday confirmed that Barca defender Thomas Vermaelen has suffered an injury to the biceps femoris of his right leg. He will be out approximately six weeks," Barca said in a statement.
"The injury happened on Friday, when Vermaelen had to come off during Belgium's UEFA Nations League match versus Switzerland. Vermaelen will miss Belgium's coming match versus The Netherlands in Brussels, as well as upcoming Barca matches."
Barcelona only have two natural centre backs in Gerard Pique and Frenchman Clement Lenglet available for a demanding run of games starting with a top-of-the-table clash against Sevilla on Saturday at the Nou Camp.
Barca visit Inter Milan in the Champions League next Tuesday and play arch-rivals Real Madrid in the "Clasico" on Oct. 28.
Barca are second in the La Liga standings, one point behind Sevilla, although they are in the midst of a poor run of domestic results, having lost one and drawn three of their last four games.
(Reporting by Richard Martin, editing by Ed Osmond)
