Lewis and Oleinik sit fourth and 11th, respectively, in UFC's official heavyweight rankings. Lewis last fought at UFC 247 in February, earning a unanimous decision over Ilir Latifi. Oleinik defeated Fabricio Werdum via split decision at UFC 249 in May.
Two days after UFC 247, Lewis appeared on Ariel Helwani's MMA Show and said he needed to resolve a serious medical issue.
"The UFC doctors know what it is, and the doctors here in Houston know what it is," he said (h/t MMA Mania's Adam Guillen Jr.). "It's just something that's been going on that I got to get addressed. It's a life-or-death situation serious."
Helwani reported the 35-year-old received a green light to compete, allowing him to line up a fight for later this summer.
Earlier this month, UFC rolled out the details of the long-awaited Fight Island, which will be on Yas Island in Abu Dhabi. The temporary base of operations will stage four events across July.
That leaves the Aug. 8 show without a firm location at the moment. Helwani reported Las Vegas is the tentative choice for now. UFC's Apex training facility is in Vegas and has been used for the company's last three shows amid the COVID-19 pandemic.