Wallace, making his fourth straight start for Delhomme, was hurt when he was sacked by Falcons defensive end John Abraham. Wallace hopped off the field on his left foot before being evaluated by Cleveland's training staff. He walked gingerly to the locker room at halftime.
Delhomme finished 13 of 23 for 97 yards and two interceptions and the Browns lost 20-10 after one of Delhommee's interceptions was returned for a touchdown by the Falcons.
Wallace will undergo further tests, but he was limping around in the locker room afterward and could be sidelined several weeks. He was 11 of 15 for 139 yards and one touchdown before being hurt Sunday.
Browns coach Eric Mangini said he will wait to see the extent of Wallace's injury before making any decisions on signing another quarterback. The only other QB currently on Cleveland's roster is rookie Colt McCoy.
Delhomme had been inactive for the Browns' previous three games after severely spraining his right ankle in Week 1. He practiced all week and was hopeful he could go against Atlanta, a team he knows well after playing seven seasons with Carolina.
Information from The Associated Press contributed to this report.