The Roberts Effect

In the second of my half term reports League Performance Regression? Yes. Here’s Why, I concluded that form has dipped in the league because the main creative players (except Sinclair) have been injured at the same time. This makes the team more predictable, and puts increased burden on Sinclair.

Of the missing creatives, Rogic has appeared more, proportionally, that last season! (It’s true – the numbers say it). It is Roberts who has missed the most game time. 14 games out injured, a 40% reduction in availability over last season.

To try and assess the impact of his absence, I compared games which Roberts started versus games he did not start. The stats are muddied a little by substitute appearances but factoring those in would have added a lot of complexity to the exercise. So, it is a binary comparison – performance when he starts versus performance when he did not start. As I only have Expected Goal and Assist data for last season for SPFL matches, we’ll concentrate on league games.

Of 74 league game Roberts has been available for since joining on loan, he has, depressingly, only started 33 matches.

Celtic achieve 4% more points with Roberts starting. Small margins but we are assessing the impact of one player.

Let’s look at chances created.

Slightly less chances created and slightly more attempted when Roberts plays. But do these chances lead to more shots?

Overall, 8% more shots are being created when Roberts starts, despite there being less shooting opportunities created. But what about chance quality? This is where Expected Goals (xNPG – Non Penalty Goals) and Expected Assists (xA) help us assess whether they are better chances or not.

And those shots result in slightly better xNPG (chance quality) and a decent 7% uplift in chance creation quality as measured.

Celtic appear to lose potency if Roberts is not starting games. The difference is a few percentage points. How significant is that? If one player is giving you more shots at goal of better quality, in a low scoring sport, I suggest you’d play him when you could.