Denmark international Agger has been playing with the injury, but complained of soreness ahead of Tuesday's Champions League draw in Porto and was consigned to the bench.

Spain midfielder Alonso missed Saturday's draw at Portsmouth and aggravated the injury in training and was also left out.

Club spokesman Ian Cotton said: "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."

Both can now expect to miss games against Birmingham City, Everton, Tottenham Hotspur, Arsenal and the home European ties with Marseille and Besiktas.

Manager Rafael Benitez has cover for Alonso with Javier Mascherano and Mohamed Sissoko able to partner Steven Gerrard in midfield. But the loss of Agger leaves veteran defender Sami Hyypia as the main replacement.

Alvaro Arbeloa can deputise in central defence, where Jamie Carragher has only just recovered from a cracked rib, while the only other alternative is young reserve team player Jack Hobbs, who has no senior experience.