I’d think the last 3 years would be a more up to date comparison even though Mitch may have still been injured last year.
Marner 22 games 25 points 1.13 P/gp
William 22 games 20 points .90 P/gp
Ok, 3 years - certainly more relevant than 8 or 9. (Slight correction to your post - Marner played 25 games.)
Regular - playoffs - drop:
Marner 1.27 - 1.00 drop 0.27
Nylander 1.08 - 0.91 drop 0.17
Marner still has more of a drop in the playoffs
Last 2 years:
Marner 1.23 - 0.94 drop 0.29
Nylander 1.13 - 0.87 drop 0.26
Last year:
Marner 1.23 - 0.43 drop 0.80
Nylander 1.20 - 0.75 drop 0.45