Juan Carlos Navarro sustained a foot injury during Spain's tournament-opening win over China on Sunday.
"It was very strong and all of a sudden,'' Spain coach Sergio Scariolo said. "It is nothing related with his former problems. It is a probably a new injury. It is plantar fasciitis, but it is probably in a different place [within the foot].
"He couldn't play. I don't know how many games he will miss.''
Without Navarro, Spain rallied behind Pau Gasol for an 82-70 win over Australia on Tuesday.
"He's not obviously too happy right now because he's injured and he couldn't play today and we don't know exactly when he's going to be able to play,'' said Gasol. "It's not the easiest situation for an athlete to go through during an Olympics.''