At the same time veteran defender Sami Hyypia is having to contend with scrutiny over his ability to continue at the highest level.
Hyypia, 34 next month and in his ninth season at the club, was a summer target of Fulham, Newcastle and Bolton, but despite Agger becoming a first choice alongside Jamie Carragher, the Finland defender was determined to see out the final year of his contract.
That has paid off quicker than he would have expected as Agger has been ruled out for six weeks with a broken metatarsal.
Spain midfielder Alonso has suffered a similar injury and will be out for a similar length of time, leaving Benitez to cope with Premier League, Carling Cup and Champions League games without two key figures of his side.
Both will be missed as Benitez has no one in his midfield who can pass the ball and open up defences to Alonso’s standards, while Agger’s absence will expose the one genuine weakness of his squad at the heart of defence.
Liverpool are still unbeaten but a Premier League draw at Portsmouth followed by the 1-1 stalemate in Portugal against Porto on Tuesday has given the Red’s so-far serene progress this season a real jolt.
Hyypia was partly to blame for Porto’s penalty in the Estadio do Dragao, where his increasing lack of pace was exposed.
The Finn, though, did not look for excuses and approached Saturday’s home match with Birmingham saying: “That result in Portugal was a kick up the backside for us all.”
Liverpool have a Carling Cup game at Reading on Tuesday, in which youngster Jack Hobbs could get his chance in defence now Gabriel Paletta has left, with the team making the trip to Wigan on Saturday week.
They are all matches Liverpool would have been expected to win before the self-doubt set in after Portugal.
“We understand we didn’t play well but still got the point which was very important, and we realise what we have to do to get a better result, it can work that way,” added Hyypia.