The Fort Worth Star-Telegram earlier reported Pachall's surgery, which now alters what had been a back-and-forth QB competition with Trevone Boykin.
Pachall injured his left arm at the end of a second-quarter running play Saturday against Southeastern Louisiana.
With Pachall out, Boykin will make his first start of the season on Thursday when the Horned Frogs travel to Texas Tech.
The silver lining for TCU is that the offense has been more effective this season with Boykin at quarterback than Pachall. Going with just one quarterback could help the Frogs be more consistent on offense, which they haven't been through two games.