Alonso aggravated the injury in training

Denmark international Agger had been playing with the injury but complained of soreness ahead of Tuesday's Champions League draw in Porto.

Spain midfielder Alonso suffered the injury in Saturday's draw at Portsmouth and limped out of training on Monday.

"Neither player requires surgery but we expect them to be out for between four and six weeks," said a spokesman.

Liverpool manager Rafael Benitez has already made 38 changes in their nine matches this season.

606: DEBATE How do you think Liverpool will cope with the absence of Daniel Agger and Xabi Alonso?

The absence of Agger could expose a weakness in Benitez's squad at the heart of the defence.

But Liverpool are boosted by the return to fitness of defenders Fabio Aurelio and John Arne Riise.