Liverpool 1-0 Southampton Match report
The Danish defender finally made the home sides pressure count when he finished off Glen Johnsons pinpoint cross after Luis Suarezs free-kick had hit the crossbar.
The result is only the fourth league win of the season for Brendan Rodgers side while Saints, who have had a revival in recent weeks, stay in the relegation zone.
But in truth it should have been far more comfortable than it actually was, which is a familiar story where Liverpool have been concerned recently.
They had 14 shots in a totally one-sided first half although Southampton could easily have had a goal of their own with Rickie Lamberts long-range shooting threatening to catch out Jose Reina.
MOVE LIKE AGGER ... Liverpool ace Daniel breaks the deadlock
The Saints striker sent an early swerving effort just wide before, in added time, his 35-yard drive had Reina scrambling back to turn ball around the post.
But those two chances book-ended what was a procession of missed opportunities for the dominant hosts.
The restoration of Lucas Leiva to the side as defensive midfield linchpin after a three-month absence with a thigh injury transformed the dynamic almost immediately.
With both Steven Gerrard and Joe Allen freed to play higher up the pitch in a 4-1-4-1 formation most of the action was concentrated in their final third.
Jonjo Shelvey, also back in the side, bobbled straight at goalkeeper Paulo Gazzaniga, Johnson flicked Suarezs pass across the face of goal, Gerrard had a shot charged down and then fired wide after a quickly-taken free-kick to Suarez.
Gazzaniga denied both Shelvey and Suarez, who made justifiable claims he had been pulled back by Jack Cork, before the Uruguay international latched onto Reinas long kick and beat the goalkeeper only for Maya Yoshida to clear the ball for a corner.
BOY DAN WELL ... Agger is congratulated by his team-mates
When Shelvey smashed an angled shot past the goalkeeper but back off the inside of the far post it appeared luck was not on their side.
However, it took a double-dose of Uruguayan intervention to eventually help break the deadlock two minutes before the break.
Saints South American summer signing Gaston Ramirez flattened international team-mate Suarez on the edge of the penalty area and although the Reds striker crashed his free-kick onto the crossbar it rebounded to Johnson.
His cross into the penalty area was met by Agger who planted a header into the top corner from 10 yards.
Southampton, having undertaken damage limitation in the first half, came out positively after the break and Liverpool struggled to reassert their dominance.
After the first-half deluge it took until midway through the second period to carve out a good opportunity.
Jose Enrique, marauding from left-back having been restored to his preferred position, exchanged passes with Suarez on the edge of the area but his toe-poke at goal was deflected wide by Luke Shaw.
The Spaniard also saw his drilled half-volley from the left of the area batted away by Gazzaniga.
BATTLE ... Luis Suarez competes for the ball with Maya Yoshida
Such was Liverpools desperation to score the second and give themselves some breathing space Suarez tried to punch in Gerrards cross.
It proved costly as his subsequent booking ruled the sides leading scorer out of next Sundays trip to West Ham, who beat third-placed Chelsea earlier in the day.
Suarez flashed a left-footed shot just wide in added time but in the end Aggers goal proved just enough to ensure they crept closer to the top four, which manager Brendan Rodgers insists remains a possibility.
It will only do so, however, if they first find more support for Suarez in the January transfer window and start converting more of their chances.
Liverpool: Reina, Johnson, Skrtel, Agger, Jose Enrique, Lucas (Carragher 88), Allen (Henderson 69), Shelvey, Gerrard, Sterling, Suarez. Subs Not Used: Jones, Sahin, Downing, Fernandez Saez, Wisdom. Booked: Suarez, Carragher. Goals: Agger 43.
Southampton: Gazzaniga, Clyne, Yoshida, Fonte, Shaw, Puncheon (Rodriguez 78), Cork (Steven Davis 70), Schneiderlin, Lallana, Ramirez, Lambert. Subs Not Used: Kelvin Davis, Hooiveld, Ward-Prowse, Mayuka, Reeves.
Att: 44,525
Ref: Michael Oliver (Northumberland).
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