Welcome to the forum.
Yes I think you need that U pipe for the exhaust to go out the muffler up top. The exhaust goes out the bottom of the engine through the exhaust ports and goes down and then back up
through the deck into the muffler. I have had that plate off of several duraforce mowers. Sometimes that U shaped pipe falls out and some times it sticks in there. As far as I can tell that flat
plate held on with the 4 bolts is the only thing that holds that pipe in there. I have read that you can use some sealer around the edge of that flat plate to seal it tight but I never have.
Good luck.
Jerry