There is nothing absolutely definitive to pin it down.
Maybe this one is kind of stupid
NHL EDGE Puck and Player Tracking Statistics for skaters including skating speed, skating distance, shot speed, shot location, and zone time.
edge.nhl.com
You compare his percentile last year to this
65th percentile was worst last year. This year he's below 50th in 3 categories.
His skating and shooting are off.
I was listening to Mirtle talk about Leafs drop in offence.
I do not recall the numbers precisely, but Matthews goals per game explained more than 50% of the team falloff.
I was listening to some other radio show. They remarked his shooting percentage was way down - about roughly half of last year
Hits/60 about 50% of last yr
Giveaways/60 2 times last yr
Takeaways/60 33% of last yr
+/- -2 this yr, +31 last yr
ppg 72% of last yr
shoot% 18.7% last yr, 10.4% this yr
Any one of these things you might ignore. Not enough data, etc.
But collectively, it is a little overwhelming.
Couple that with what Auston & his coach have said.
No one is imagining it. He is not 100%
There is clearly something significantly wrong.