Thomas will undergo surgery to repair his broken right forearm, Lindsey Pallares of the 49ers' official site reports.
Thomas broke his right forearm in the first half of Saturday's preseason loss against the Titans. The recovery from his surgery will keep the 2021 third-round pick sidelined for multiple weeks. With Thomas on the shelf, the 49ers will try out Renardo Green, Samuel Womack and Rock Ya-Sin to serve as the second starting corner opposite Charvarius Ward.