Freese suffered the injury on June 5 after stepping awkwardly on third base, and the rookie said that he has been playing through pain ever since. In 18 games since the injury, Freese has hit .220 with a .264 on-base percentage and .240 slugging percentage.
"It got better for a couple days, but I just wanted to get back out there and try and tough it out," Freese said. "I thought it would gradually get better as I was playing. It just never did.
"Hitting was tough, getting of my backside, it just wasn't helping."
Manager Tony La Russa said he expects that Freese will be back within the 15 days.
"We have been wrong before, but they are telling me we should have him by the All-Star break," La Russa said.
Salas has made four appearances with the Cardinals this season, tossing 4 1/3 scoreless innings with four strikeouts.