The Liverpool manager was already reeling from a 4-0 defeat at Tottenham that included red cards for midfielder Charlie Adam and defender Martin Skrtel when he was hit by a shock diagnosis on Agger's latest injury setback.
Apart from missing Wednesday's Carling Cup tie at Brighton and Saturday's visit of Wolves, Agger has also been ruled out of the derby meeting with Everton at Goodison Park and will struggle to make the October 15 showdown with Manchester United at Anfield.
Out of action: Agger faces four weeks on the sidelines with a fractured rib
It is worrying news for Dalglish, after signs of defensive frailty at White Hart Lane, and for Agger, who was helped off in obvious discomfort against Tottenham.
Dalglish at least has the consolation of being able to call on skipper Steven Gerrard, who returns to the squad after a six-month lay-off with his latest groin problem.
Up for the cup: Brighton's Gus Poyet
The midfielder is eager to make up for lost time and help revive his side's fortunes after successive away defeats at Stoke and Tottenham. Dalglish must decide whether to restore him to the starting line-up or ease him back via the bench.
Defender Martin Kelly is also ready to return after shaking off a hamstring injury.
Brighton boss Gus Poyet admits his side will need to play the 'perfect' game if they are to beat Liverpool.
Poyet, who will be marking his 100th match in charge of Brighton, said: 'We need to be at the top of our gameand be a bit lucky, and maybe if Liverpool aren't then we've got a chance.
'We have to make sure we are better than them on the day and take our chances. You need practically the perfect game to beat them, but we are going to try.'
Poyet could hand a debut to former Spain midfielder Vicente, who joined as a free agent earlier this month,while assistant manager Mauricio Taricco, 38, is set to deputise for the suspended Marcos Painter at left back.
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