The official advice from NutriBullet is that if the rubber ring falls out, then you should buy a new blade assembly.

There may just be something wrong with the gasket and it is possible to buy a replacement gasket. However, this does not always mean that the entire blade assembly needs to be replaced. If your NutriBullet gasket falls out continuously, it normally means that it has become stretched and is no longer a snug fit. What to do if Your NutriBullet Gasket Falls Out

NutriBullet recommends that if the seal is damaged, you should buy a new blade assembly. You can buy replacement gaskets for your blade assembly, but the official advice from NutriBullet is to not replace this part. This can also happen over time after regular use, so make sure to inspect regularly for damage. If you have noticed that your blender has been leaking from the top, then check to see if there are any cracks in the rubber seal before buying a new one. If this ring becomes damaged, then it will cause leaking from your NutriBullet’s lid. The seal or gasket is a small rubber ring that sits between the cup and lid. Make sure that all parts are locked together tightly before you flip the cup upside down. If the blades are not fully tightened into place then that could cause a leak at best and outright breakage at worst. The blades must lock into place when attaching them to the blender cup before turning on your machine.

One common reason why your Nutribullet may be leaking is that you are not using it correctly.

Nutribullet glass cup