Here's the thinking I suspect the Rams are having.
First, there is no guarantee that OBJ is coming back.
Second, the 3rd and 4th WRs are going to play big roles in this offense. Sending Jefferson away essentially now has the Rams using their 4th and 5th WRs.
Third, injuries happen. If Kupp or Robinson were to go down even for a couple of games, the Rams could come nearer to replacing their production with Jefferson more than any other WR on the roster.
Fourth, we always overestimate how much we can get in trade for a player. My guess? A 5th rounder, but that's only if his knee proves to be healthy by his playing and proving it. That's not good value for what he brings to the Rams offense.
I do think they will not rush him back. He probably won't be ready for the first 2 or 3 weeks of the season. McVay is conservative concerning injuries. That means they're going to get a good look at some of these young players. But even more, despite all the happy talk about how well his surgery went, that knee might become a permanent issue. And I can't imagine that some team would want to invest in him without a better reading of his medical condition.