With Hovland battling injury, what is Ryder Cup envelope rule?

Viktor Hovland's neck injury could invoke the rarely used envelope rule at the Ryder Cup in New York. Hovland withdrew from the afternoon fourballs due to the injury and is undergoing an MRI scan to assess its severity. If he can't play his singles match against Harris English, the envelope rule may be implemented. Europe currently leads the Ryder Cup with an 11½-4½ score going into the final day.