In a medium bowl, toss together the rhubarb, sugar and cinnamon. Add half of the bread cubes and toss lightly to distribute. Transfer to an ungreased 2 litre microwave-safe baking dish. Top with remaining bread cubes. Drizzle melted butter over the top.
Cook in the microwave on full power for 12 minutes, or until the rhubarb is tender. Serve warm.