Barcelona coach Pep Guardiola said he was concerned over an injury to Lionel Messi that forced the Argentine forward to be substituted during their 1-1 La Liga draw at Athletic Bilbao on Saturday.
The timing of the news couldn't be worse with new league leaders Real Madrid preparing to visit Nou Camp next Sunday, with their star signing Cristiano Ronaldo hopeful of a return from his own lay-off, while Barcelona's Champions League hopes are on the line in midweek.
Messi was replaced in the 75th minute by Bojan Krkic at San Mames with what appeared to be a torn left thigh muscle, Barcelona said.
"We took him off because he was in pain. We'll do more tests tomorrow but the outlook is not good," Guardiola said after the match.
Sweden striker Zlatan Ibrahimovic also missed the game with a thigh strain, while Eric Abidal and Yaya Toure were laid low with the HIN1 virus and Rafael Marquez also missed the trip to Bilbao with suspected swine flu.
The European champions host Inter Milan in the Champions League on Tuesday when a defeat combined with a win for Rubin Kazan would put them out of the competition.