American Soccer Analysis earlier this week announced the creation of a new metric by which to judge players' contributions to a game, known simply as "goals added."
Essentially, the metric takes data from every single on-the-ball action during a game, and measures how each action by each player changes the team's probability of scoring or conceding a goal during that sequence of play. From that data they calculated the most influential MLS players at every position and at the top of the list for center backs was Philadelphia Union defender Jack Elliott.
The metric does not factor in whether or not a player actually contributes a goal or an assist during his action — rather only if he increases his team's probability of scoring.