Just an opinion. What I take from being more "pro ready" is the conglomeration of physical skills, experience (who they played and in what division), system they ran, etc etc etc - gives them a leg up on the others to 'adapt' to the Pro game.
Keenum vs Goff vs Mannion - so far, and to me. I'm happy with Keenum. As I've said many times... his debut looked terrible to me but he didn't quit on himself and by season's end became, IMHO, a solid NFL QB is not a spectacular one, far from spectacular IMHO but none the less capable. I think his work ethics are paying big dividends.
Goff - again IMHO, clearly has the best arm of the three. There isn't a pass he can't throw. He's been under the gun so far (didn't see this last game but I did watch his hi lights), and throwing under pressure a lot... so what I've seen I would expect... he has yet to disappoint me, IOW.
Mannion, as you know, I think is 'underrated' - and by nearly everyone. Doesn't have the arm Goff has, and certainly not the experience Keenum has, but, and like Goff.. his ceiling seems high to me and his work ethic and attitude ready to get him there.
Goff and Mannion I believe have much higher ceilings than Keenum, neither is as 'ready' as Keenum. And Keenum is far MORE ready than many recent, but now gone, starting QB's for the Rams. - JamesJM