The 49ers placed Thomas (forearm) on injured reserve Wednesday, Nick Wagoner of ESPN.com reports.
Thomas is underwent surgery to repair his right forearm, which he broke during the 49ers' preseason opener against the Titans. It's unclear when he'll be cleared to return to practice, but Thomas will be forced to miss at least the first four games of the regular season while on IR. The earliest he can return is Week 5 against the Cardinals on Oct. 6.