The St. Louis Cardinals are rumored suitors for the likes of Jake Arrieta, Yu Darvish, and Chris Archer. In order to accommodate a starting pitcher to a rotation currently consisting of Carlos Martinez, Michael Wacha, Adam Wainwright, Miles Mikolas, and Luke Weaver, one of the younger pitchers would either need to be traded, moved to the bullpen, or sent back to the minors for additional seasoning.
I do not believe the St. Louis Cardinals would move Luke Weaver to the bullpen if his spot is taken in the rotation.