The Panthers opened the 21-day practice window for Thielen (hamstring) on Wednesday, Darin Gantt of the team's official site reports.
Thielen can be activated from injured reserve at any time within the three-week window, making him eligible to play as early as Sunday's game in Denver. The veteran wideout has sat out Carolina's last four contests due to a right hamstring strain. Once fully cleared, Thielen should reclaim a regular role in three-receiver sets alongside Diontae Johnson and Xavier Legette.