Thomas suffered a broken right forearm in the 49ers' 17-13 preseason loss to the Titans on Saturday, David Lombardi of The Athletic reports.
Thomas left shortly before halftime Saturday and finished the contest with one tackle. The 2021 third-round pick will likely require a multiple-week absence that could extend into the first few games of the regular season. With Thomas sidelined, Renardo Green, Samuel Womack and Rock Ya-Sin are the top candidates to push for the second starting job at corner opposite Charvarius Ward.