Denver signed Mack to the active roster off the 49ers' practice squad Monday, Adam Schefter of ESPN reports.
Mack is set to fly with his new team to London on Monday in preparation for a Week 8 matchup against the Jaguars. The veteran's chance in Denver comes following an ankle injury to Mike Boone, the severity of which has yet to be determined. If Boone can't go versus Jacksonville on Sunday, Mack could see work in a backfield led by Melvin Gordon and Latavius Murray.