Barcelona defender Andreu Fontas tore a ligament in his right knee during their King's Cup victory over Osasuna on Thursday, and will miss the visit of Real Betis on Sunday.
He joins long-term injury victims David Villa and Ibrahim Afellay in the treatment room, along with forward Pedro who suffered a hamstring strain in the same match.
"I don't like signing players in January... we are going to rely heavily on the youth team," Guardiola told reporters in Pamplona. "I believe in them completely. They better prepare themselves."
Third-placed Valencia host Real Sociedad on Saturday, and the visitors, 16th, arrive after letting slip a 3-0 advantage to lose 6-3 on aggregate to Real Mallorca in the King's Cup during the week.
"The team are in debt to the fans and they need to give them a lift again and soon," Sociedad President Jokin Aperribay said on a visit to their training ground. "We cannot accept what happened in Mallorca."
Sevilla's cup exit during the week and three straight defeats in La Liga have put the pressure on coach Marcelino and his players for the visit of Espanyol on Saturday.
Atletico's Diego Simeone and Villarreal's Jose Molina, former team-mates at Atletico in the 1990s, drew their first games in charge last week.
