If I'm not mistaken, Sammy Watkins was acquired the day before the Rams first pre-season game last season and didn't get on the practice field until a couple days before the 2nd. This is in stark contrast with Robert Woods and Cooper Kupp who were acquired/drafted in time for a full mini-camp. And with both of those guys being high-IQ football types, Goff established an early on-field rapport with them. Same could likely be said for McVay working with Woods and Kupp in the classroom. I distinctly remember reading an article detailing how Woody called McVay on his way home after practice to discuss how to correct an on-field screw-up that had occurred earlier in the day. Unless there was some serious ying & yang going on, there was no way that Watkins was going to achieve that same dynamic.
But that shouldn't be the case with Cooks. He should have more than enough time to get with Goff, Woods, and Kupp to iron out the timing and routes.