Well, this is arguably the best situation Stills has ever had. I mean, he played with Brees, but he was pretty one dimensional then, and had to contend with Graham, Colston, and Cooks.
In theory, Stills could/should be more productive than Fuller was. He's almost as explosive, and has much better hands. Its just a matter of getting Watson's trust. Fuller arguably should have been fantasy's #1 overall WR these past 4 games, if he didn't drop like 4 TD's, including 2 over 40 yards. So, there is certainly room for Stills to be a high-end WR2, its just a matter if he will.
I wouldn't be totally shocked to see him get heavily involved early, as Oakland may sell out to stop Hopkins, and force Stills/Coutee to beat them in Fuller's absence. Anywhere from 4-60 to 8-150-2 seems within reason to me.