Alfaro is highly overrated, both Rupp and Knapp outhit him in AAA ball at the same age.
He's got a strong arm, but is erratic otherwise defensively, he hasn't stood out this year in Lehigh.
Grullon is 21 and now at Reading, he has the same defensive tools.
If they need to carry three catchers they can do so b/c Knapp can backup at other positions (i.e. you can pinch hit with a catcher), so you don't need a 1B/OF type on the bench. This could buy them time to evaluate Knapp as a potential starter, Alfaro as a potential anything, and Grullon's development.
But it makes no sense to weaken the ML team in 2018 for a marginal return.
For one thing, Rupp with three ML years of experience makes more sense with a young starting staff than Alfaro as a rookie.
If you're really hellbent on trading Rupp, I'd move Alfaro as well and bring in a veteran catcher, shouldn't be too hard to find one who can give you .660 OPS and good defense and experience handling pitchers.