The Twins have near-majors non-roster offensive talent in the minors (Austin Martin, Brooks Lee) but no non-roster pitching talent above A-ball.
If the Twins trade Arraez they are counting on one of Lewis, Larnach, Kirilloff, Wallner, Julien, Lee or Martin to step up and fill a spot in the lineup. They don't have 7 potential prospects to fill a spot in the rotation - just SWR and Varland.