I was indirectly asked about Liverpool's center-back pairings on Twitter this morning. Which seems relevant, as Lovren's signing is apparently imminent, and Agger's perpetually rumored (emphasis on "rumored") to be moving on. So here's a quick comparison from last season's league fixtures.
There are, of course, a handful of caveats.
You can discount three of those immediately: Toure-Sakho, Toure-Skrtel-Agger, and Skrtel-Agger-Sakho, each appearing for less than 25 minutes at the end of matches, with Rodgers making substitutions to either shore up the defense or, in the case of Toure-Sakho in the last 18 minutes of the home loss to Southampton, remove a defender for an attacker.
Of the four pairings (well, three pairings and one trio) that played in six or more matches, the Skrtel-Sakho duo appeared in the hardest fixtures, by far.
The average league position of the opponents Skrtel and Sakho faced was 9th. Most notably, they played the full 90 minutes in both matches against City and Chelsea, Liverpool's two closest competitors, with Toure left on the bench for all four, and Agger left on the bench for three of the four. The opponent's average league position was 11.5 for Agger and Skrtel, 11.1 for Skrtel and Toure, and 11.5 for Toure-Skrtel-Sakho. In addition, nine of Skrtel-Agger's 13 matches were at Anfield. Just five of Skrtel-Sakho's 12 were.
Part of that was due to each player's availability – both Sakho and Agger missed matches they probably would have started in if fit at times last season – but Skrtel-Sakho did seem Rodgers' most-trusted pairing.
However, in their six matches against top-eight opposition (away to Everton, home against Tottenham, home and away against both United and Southampton), the Skrtel-Agger pairing conceded just four goals – three at Everton, one against Southampton – keeping clean sheets in the other four wins. So it's not as if they played badly against tougher opposition. You know, aside from allowing three goals at Everton. Although all four of those goals at least started from a set play, if not came directly from said set play. Which isn't good.
I am slightly surprised we never saw Toure-Agger again after clean sheets in the first two matches, but the pairing allowed more shots per 90 than any other CB duo that played at least one full match, and, to be fair, those wins and clean sheets came against Stoke and Villa.
The full list of each grouping's opponents (click to open in new window as it's necessarily quite wide):
I've complained about Opta's definition of a defensive error (doesn't necessarily include own goals or penalties conceded), but as it's the best I've got at the moment, a list of who partnered whom when each defender made a defensive error.
Skrtel (4): Toure and Sakho, Agger, Toure, Sakho
Agger (3): Skrtel, Skrtel, Skrtel
Toure (3): Skrtel, Skrtel, Skrtel
Sakho (2): Skrtel, Skrtel
Unsurprisingly, Skrtel's on the pitch when Agger, Toure, and Skrtel made all their errors. Because Skrtel was almost always on the pitch. He played 3221 minutes (out of 3420 possible) last season. Sakho played 1477, Toure 1447, and Agger 1420. Skrtel committed his errors no matter who he partnered. So, yeah, that doesn't help much.
Long story short: if you account for the strength of opposition, there doesn't seem a whole lot of difference between Skrtel/Agger and Skrtel/Sakho; there's room for improvement no matter which center-backs are on the pitch; and Skrtel-Sakho appears to be Rodgers' preferred pairing (if, of course, Sakho can stay healthy). For now.