I'm in the bullpen camp, too, unless Keuchel and McCullers are seriously injured and this isn't a conservative approach to manage small injuries.
Look at what Cleveland did last year with Kluber and scrubs in the rotation. Let's say we have Keuchel, McCullers, Morton, Musgrove/Martes/Fiers as our starting 4. If any of those guys get off to a rocky start in one of their starts we go straight to Devo. Then all of a sudden it's a bullpen game because with our offense we will never be out of a game. Cleveland did the same thing last year with Andrew Miller.
The SP market is too underwhelming to trade Tucker, Bregman, Whitley, or Perez in my mind. I would consider trading Perez or Whitley for a big time reliever, though.