According to the Barcelona web site:
Over the last few weeks, FC Barcelona manager Josep Guardiola has been getting pains in the lower lumbar region, and the problem has worsened in recent days to the extent that he was unable to attend training this morning.
Assistant coach Tito Vilanova will handle the pre-game press conference and will take charge of the team if Guardiola is unable to make the trip to Valencia.
While Guardiola was absent from training, Xavi Hernandez continued his recovery from a leg injury. Defender Carles Puyol and goalkeeper Victor Valdes again trained separately with next week’s Champions League match against Arsenal in mind.
Barcelona goes to third-place Valencia with a seven-point lead over Real Madrid.