One way to look at is that the Rams got their 6th round pick back that they used to draft Gaines.
So, essentially a 2nd round pick for a star #1 WR. Sorry, but that is just and excellent pick up.
Sure, injuries can't be discounted. But clearly he is healthy and add in that he's only 24. Not too many miles on that body.

I just don't know how you do any better than this with a 2nd round pick.

If Watson stays reasonably healthy and does what he'll probably do, Rams aren't about to let him escape.