I get where you're going, but you're looking at it the wrong way IMO.
Instead of looking at his statistical output in terms of raw numbers (goals, assists) and basing your assessment of his potential based on that, you should base your assessment of his potential on the metrics.
The headline is that Celtic are paying £3.25m for a midfielder from Motherwell who scored 15 goals last season. On the face of it, 15 goals last season is a very impressive figure.
BUT
Three of those goals were penalties. While penalties still have to be scored, they're not really a good measure of your worth as a player, because they're high percentage scoring chances and they don't come from open play. And he is unlikely to be taking penalties for Celtic.
Therefore, Turnbull scored 12 goals from open play. Again, still an impressive number, but not as impressive as 15.
Digging into those twelve goals, using an xG model, a significant amount of them were either long range shots that were deflected, or shots that the keeper should have saved. His xG total means that from the chances Turnbull had, you would expect him to score only 4 goals. So he got very, very lucky to score 12 goals from those opportunities. Even if he was a very good finisher, you would only expect him to put away 4/5/6 goals. 12 is a stastical outlier that simply has to regress to the mean.
So the headline is that Celtic are buying a 15 goal-per-season midfielder. But if Turnbull hadn't got lucky, Celtic would be buying a goal-scoring midfielder who scored 4 goals and 3 penalties. For £3.25m. From Motherwell.
And FWIW, Turnbull is a goal scoring midfielder, because the metrics also show:
His defensive stats are poor compared to other SPFL midfielders (doesn't win the ball back, doesn't make interceptions, average at closing down)
His playmaking stats are league average at best (average for xA, average at dribbling, average for giving the ball away).
The source:
