You will most certainly need support. The corners of the rectangular segment will droop substantially during the print. The overhang at the area near the cylinder may be only a couple of millimeters, but the corners are much farther away.

Some printers do better than others in creating support that's easily removed. Your choice of slicer also will have much to do about the results. Prusa Slicer along with a Prusa printer works great. Another example of which I am aware is Matter Control on a Robo3D R1+ printer. I'm happy with Simplify3D on a Sigma R16. Your mileage may vary; California mileage will be less.

If you place a cube shape of narrow dimensions at each corner at the height of the underside of the rectangular shape, the overhang becomes a bridge, which may result in a smoother underside. Angus from Maker's Muse has a video on the YouTube showing how that can reduce/eliminate the need for difficult to remove supports. At least in your case, your supports will be easy to remove. They aren't embedded in tiny holes or other cavities.