I don't trust a rookie (especially Prato or Severino) to immediately come in and produce at the plate. Both Castro and Farmer can field well enough to hold down 3B and in a platoon role they can provide enough offense for the short term. I also don't want to start the clock on Lee if they're going to send him back down when Lewis returns.
I would move both Severino and Lee to St. Paul. I expect the Twins will make that move after the trade deadline, assuming they are both still in the organization.