I think it's not an apples-to-apples 'how do we replace Watkins'. It's about what the offense does
differently in 2018 vs. 2017. They have the steady influences of Goff, Woods, Kupp and Gurley. That's a heck of a start... And all those guys are refining their mastery of the offense vs. learning it. Not insignificant.
The thinking can go to Watkins touches and all that. And what he gave them, while being their 4th leading receiver, was good. And part of his value was in the attention safeties gave him over the top. But it's not like him leaving is going to drop a safety back into the box. And the offense can evolve to make up for it. Personally I look at Everett (16 catches) and Reynolds (11) and think "can these guys develop further and combine for 70 catches in bigger roles?" Yes, I do. Again I posted earlier that I do not think McVay is satisfied getting 49 catches from his TEs after having three years in DC with TEs combining for 100+ catches/year.
Something is going to look different in their offense next year. And frankly, good... because DCs are going to be studying them and trying to catch up. So it's really not a matter of 'who do we sign/draft/plug in to be Sammy Watkins?' You don't.
Evolve or die...