Lohse, who was placed on the 15-day disabled list Thursday with exertional compartment syndrome, will have the surgery performed by Dr. Stephen Shin in Los Angeles.
Lohse was examined by Shin on Wednesday at the Kerlan-Jobe Orthopedic Clinic in Los Angeles.
"It explains everything I've been feeling throughout the last year," Lohse said. "It explains why it's such a hard thing to diagnose."
Exertional compartment syndrome is an uncommon, exercise-induced neuromuscular condition that causes pain and swelling in legs or arms and can be difficult to identify.
Lohse will have fascial release surgery performed on his forearm. There is no timetable for his return.
Lohse. who is 1-4 with a 5.89 ERA, has not pitched since Saturday, when he lasted only 3 1-3 innings in a 10-7 loss at the Los Angeles Angels. Lohse said he experienced fatigue and cramping in his forearm in that start, similar to what he's been feeling since last season.
The move to the disabled list is retroactive to Sunday.
Lohse joins right-handed starter Brad Penny on the DL. Penny was placed on the list Saturday because of a back strain.
St. Louis purchased the contract of right-hander Fernando Salas from Triple-A Memphis to replace Lohse on the roster.