The problem with Kupp earlier in the year had more to do with he and Matthew still tweaking the rules in the option routes as teams have learned some of the tendencies. Since then, yes, he's not seperating as well because his elite quickness has been diminished so his body isn't always responding as it was before and he has to adjust how he runs his routes. He's also regularly slamming his body into DE's, LB's, and anyone else he's called to block.
In a couple years he may be our '99 Ricky Proehl and though he will likely never win another triple crown. he remains an essential part of an offense that has a chance this year.
Cooper Kupp is a physical receiver who has been blocking like a tight end for years now. His mind has pushed his body to its very limits and the body can only take that abuse for so long. He is, and will continue to adjust his game to maximize his effectiveness and I personally wouldn't bet against him being the stud he is when do or die time comes his way. That's the Cooper Kupp I trust hasn't diminished one iota.