If Ashton Jeanty is ruled out for Week 1, yes, Mike Washington Jr. would almost certainly start — he's the clear RB2 and has already been tabbed as the primary bell-cow in Klint Kubiak's zone-run system if Jeanty misses time (from the Mike Washington Jr. page). He also had a strong preseason, rushing for 100+ yards across two games.
That said, the Raiders' brass (Rapoport and Schefter) have stressed Jeanty's sprained ankle is not a long-term issue, and he's expected back to practice sooner than initially feared. So Washington Jr.'s window as a starter may be limited to a week or two at most.
On FantasyPros, an analyst put it plainly: "Genty [Jeanty] goes down... Mike Washington Jr. is the clear and definitive handcuff" — . He's a strong flex play if Jeanty sits, but don't expect a multi-month takeover.