Here's a quandary between tactical ideas and pragmatic results. Jürgen Klopp likes to play a high defensive line, which places a premium on central defenders who are both quick and have strong ball possession and distribution skills. However, since Klopp's arrival (and arguably long before it), opposing teams have battered Liverpool's defense with aerial attacks. This weakness suggests a need for a different type of center back - one who is tall, reads the game well, and is good in the air. Occasionally the quick, possession-oriented center back comes in the same package as the aerial dominator. But, more typically, those two sets of strengths are found in different players.
Among the current center backs, Kolo Toure is almost certainly on his way out due to his advanced age, and he has hinted as much publicly. That leaves Mamadou Sakho, Martin Skrtel, and Dejan Lovren as the experienced center back choices. In addition, youngsters Joe Gomez and Tiago Ilori should both be available by the start of next season.
When healthy, Mamadou Sakho perfectly matches the tactical blueprint for an ideal Jürgen Klopp center back. He's not incredibly quick, but quick enough. He is very aggressive, and prefers to play a high line. Despite the ridiculous complaints that he is "awkward on the ball," Sakho is by far the best passing center back on the team. Sakho's ability to send incisive passes to attacking players is unique among LFC central defenders. Indeed, this skill makes his presence arguably just as valuable to the team's attack as it is to the defense.
Unfortunately, despite all of these strengths, Sakho is nonetheless far from perfect. First, he is too often not healthy, as is currently the case. Since returning from his latest injury - to a knee - Sakho has been far from his best, and the team has suffered for it. This knee injury is merely the latest in a long line of fitness problems that have plagued the French defender since he joined Liverpool in 2013. Second, even when healthy, Sakho is relatively weak in aerial battles. Though he is far from being solely to blame for LFC's vulnerability on this front, he bears at least his fair share of responsibility.
Skrtel is in some ways the opposite of Sakho. Although he is not dominant in the air, he is better than average. Last season he won 65% of his aerial battles, compared with 57% for Sakho (although this season they are running a much closer race, with Skrtel at 66% and Sakho at 64%). Skrtel prefers to play in a deep position, and he excels at covering as the last defender. But he's prone to silly fouls and too often loses his marker on set pieces. He is an average passer for a defender, and he generally settles for the easiest pass available. He is not particularly comfortable playing a high defensive line, which renders him tactically unsuitable for Jürgen Klopp.
Then there is the curious case of Dejan Lovren. Like Sakho, he is an aggressive defender. But he is more prone than Sakho to getting caught completely out of position, and he has frequently found himself watching from afar as an opposing striker latched onto a through ball and placed a shot past a stranded Simon Mignolet. Lovren fancies himself as a passer, but his attempts at crossfield passes and long balls often go astray. He is also far likelier than Sakho to pass the ball back to Mignolet, rather than pushing it forward. In short, Lovren is far closer to Skrtel's skill level as a passer than he is to Sakho.
Nonetheless, unlike his centerback partners, Lovren is very strong in the air. He has won 78% of his headed duels this season. He has also been beaten for goals on crosses and set pieces, but not so often as his mates.
Center back needs: Klopp should bring in one new center back when Toure leaves, and he should give Gomez and Ilori opportunities to compete with the others. Gomez has the physical tools to fit Klopp's system perfectly, and Ilori could develop that way as he gets physically stronger.