Liverpool duo Daniel Agger and Xabi Alonso are out for up to six weeks with metatarsal injuries.
Agger suffered his problem while playing for Denmark last week, and Spanish midfielder Alonso also picked up his injury on international duty.
Both players were able to feature in last weekend's 0-0 draw with Portsmouth, but missed Tuesday's UEFA Champions League game with Porto.
Alonso was taken off against Pompey and was then forced out of training early ahead of the 1-1 draw with Porto in Portugal.
Scans on the injuries have now shown that Agger and Alonso have suffered stress fractures, and are out for a minimum of four weeks.
Club spokesman Ian Cotton told Liverpool's official website: "Neither player will require surgery, but we expect them to be out of action for between four and six weeks.
"Xabi Alonso has suffered a stress fracture to the fourth metatarsal in his left foot and Daniel Agger has suffered a stress fracture to the second metatarsal in his right foot."
Reds boss Rafa Benitez should have sufficient options to cover the absence of Alonso in midfield, but the injury to Agger will be a blow.
Jamie Carragher and Sami Hyypia are now the only senior recognised centre-backs available for selection, although Benitez has previously admitted he could shift Alvaro Arbeloa inside from a full-back role.