Where art thou, William Saliba?
Has the absence of William Saliba (and consequent presence of Rob Holding) affected Arsenal’s title hopes?
As Manchester City takeover as the favorites to win the Premier League title (SkyBet currently have the odds of Manchester City winning the league outright at 1/16), a large multitude — my friends and what I see on social media — have blamed the perceived absence of their titan defender William Saliba for their recent poor run of results. Arsenal had been on top of the table for majority of the 22/23 Premier League season but surrendered that to Manchester City with a dire run of results (last 5 games: WDDDL). Do Arsenal fans have a point? How much of this collapse can be pinned on the missing player?
It is historically difficult to quantify an individual defender’s true impact on a team — metrics such as clean sheets, tackles, interceptions, blocks and such alike all have caveats. Take tackling for example, measuring a player’s defensive proficiency by the number of tackles attempted and completed is rebuffed by one of the greatest defenders to play the beautiful game.
“If I have to make a tackle then I have already made a mistake.” — Paolo Maldini
One of the greatest midfielders to play concurs:
“I don’t think tackling is a quality. Tackling is a last resort, and you will need it, but it isn’t a quality to aspire to.” — Xabi Alonso
Standard defensive stats are usually skewed by team composition and team style. A team who dominate possession will have less tackle attempts, a team sitting in a low block will have more clearances and blocks, clean sheets are usually symptomatic of good team structure, good goalkeeping and a little bit of luck. Conundrum.
“It is much easier to collect data on forwards. This is because attacking is generally an individual enterprise, whilst defending is a collective responsibility. A player scores a goal; a team concedes a goal.” — James Tippet, The Expected Goals Philosophy
The book Football Hackers by Christop Biermann posits an interesting approach to this conundrum. An approach inspired by the game of basketball. The book details a tale dating back to 2001 when the Dallas Mavericks basketball team signed a virtually unknown center. How unknown?
“This was basketball’s equivalent of Liverpool buying a Hudderfield reserve player and awarding him Mohammed Salah-type wages.” — Biermann, Football Hackers
The Dallas Mavericks signed the player because of his plus-minus data. A player’s plus-minus data is the net points won and lost whilst the player is on the court. Although that particular acquisition fell very short, Mark Cuban, the owner of the Mavericks is still a huge advocate of the use of this metric to quantify player importance,
“This player who always wins 50–50 balls, barks defensive instructions at his team-mates and hunts down the opposition — the classic stats don’t reflect his value. But plus-minus does.” — Mark Cuban, Owner of the Dallas Mavericks
Applying this to football, an equivalent metric to plus-minus would be Expected Goal Difference (xGD). More specifically, what is the discrepancy between a team’s (Arsenal) total xGD vs xGD with the player in question (Saliba). This will both explain and quantify Saliba’s impact on the team.
The figure below displays the discrepancy between Arsenal’s total xGD vs xGD when Saliba plays. Their expected goals differential is lower with Saliba in the side, which isn’t to say Arsenal are better without him — but assuming an accurate xG model, they’re certainly not much worse off without him. Leave Rob Holding alone.
If you’ve made it this far, any and all scrutiny is not only welcome but encouraged. For contact, email me: remiawosanya8@gmail.com