I have him as a top 10 WR in the NFL, probably top 5.
Since 2017, no receiver has gotten open more than Kupp or gained yards after the catch at a higher rate than the Rams receiver.
If I am reading this right, he is better than any player in the NFL in both:
1) Percentage of time getting open (Somewhat subjective I am sure, but he obviously is great at that)
2) Yards after the catch
So if you have a guy that gets open better than anybody in the world and gets more yards after the catch, I think that means he is a great player.
