Be careful driving at speed until you replace it. The part behind the missing area is flexible and can act like a sail, especially at freeway speeds. As it scoops in more air, it gets pressed down, the sides weaken and eventually rip, and soon you could be scraping up all the road debris with it... ask me how I know
If you can't replace it soon, I would recommend thinking about taking it off, otherwise it will probably break at the most inopportune time. And you can't really duct tape it back together as a temporary fix.
Part #20 in the diagram should come with two of #24 (Bowden cables) and two of #25 (rubber strips) already attached.