When you purchase a new bottle, the plastic ring is connected to the lid by a perforation that breaks when the bottle is opened for the first time. So, as long as the plastic ring is intact upon purchase, it’s safe to drink.
Just fold the flap backward and use it as a dipping plate for your ketchup (or whatever sauce is your favorite). It’s a useful design that’s not featured in McDonald’s marketing materials, but once you try it, you’ll be a lid-dipper for life.