Trading Miley seems risky given his floor and reliability and the Red Sox desire to compete, and without a replacement I'm not sure I would be looking to trade him.
That said, his upside is also limited, and his abilities are kind of duplicated by Johnson and Owens. So if you can move him for some pieces that fill in the player development gap (namely pitchers that are 1-2 years from the majors) then it could make sense from an organizational standpoint. But I would be more into that if I were sold on Kelly or if Johnson wasn't coming back from injury. And, you could also wait until the ASB or next off-season to make that kind of move.
If for some reason the Red Sox were already committed to giving Joe Kelly a starting job, and Miley would be displacing Rodriguez and his options until there were injuries, you could more easily justify a trade. But I doubt they are in that position.