Downs suffered a high-ankle sprain during Wednesday's practice, Jeremy Fowler of ESPN.com reports.
Per Fowler, the initial outlook is that Downs could miss about a month, but that's not yet a concrete estimate, as the Colts are currently working to establish a timeline for the receiver's return. In any case, Downs' Week 1 status will be worth monitoring closely, and if he's out or limited early on this season, Adonai Mitchell and Alec Pierce could see added opportunities alongside Michael Pittman.