Teams will try free agency first. A Berrios trade is unlikley to happen until many of the free agent pitchers are signed. Then they could opt for Berrios because of a lack of options.
The problem for the Blue Jays is that if they trade him late in the offseason, they are stuck filling in the 5th starter role with the leftovers.
If they are confident they can move him, then it is best to gamble on signing the 5th starter sooner than later, and then dealing him afterwards.