Short answer: It's unclear — he's questionable for Week 1 after spraining his ankle on Aug 23.
Jeanty left Raiders practice after an awkward landing that initially looked like a catastrophic knee injury, but Ian Rapoport and Adam Schefter confirmed it's a sprained ankle, not long-term. Rapoport said on Aug 25 that "there is a chance that Ashton Jeanty may not be on the field for Week 1," though head coach Klint Kubiak told reporters they're "really excited to get him back." The ankle sprain is described as "more low than high," which is a good sign recovery-wise, but no official timetable has been set (from the Ashton Jeanty page).
If he misses Week 1, Mike Washington Jr. — who's had a strong preseason — would be the lead back in Vegas.
On Fantasy Football Today, the crew noted the general sense of surprise that the injury wasn't worse than initially feared — .