Volquez, who isn't traveling with the team, has been on the 15-day disabled list since June 2 with elbow tendinitis. He pitched just one inning after a return from the DL with back spasms.
The extent of the inflammation in Volquez's arm wasn't detectable in the first MRI because swelling made the picture less clear.
"It just means it will take a little longer, but we still expect a full recovery," Reds general manager Walt Jocketty said. "You have to make sure it heals completely. Those things generally do."
Team medical director Dr. Tim Kremchek will examine Volquez again on Tuesday. The right-handed pitcher had been long tossing and was up to pitching on flat ground last week. He had hoped to progress to throwing off a mound, but that is on hold.
The Reds aren't expecting to have the starter back any time soon. Last season, Volquez was a 17-game winner and an All-Star. In nine starts this season, the 25-year-old is 4-2 with a 4.35 ERA.
"With a starting pitcher, you need time to build back up and have rehab starts," head trainer Mark Mann said. "You're probably talking beyond the All-Star break."
The Reds still need a starter to fill Volquez's rotation spot for Saturday at Cleveland. His first replacement, Matt Maloney, already was optioned back to Triple-A Louisville last Saturday.
No pitcher was formally named, but all indications are that Homer Bailey will be brought back up from Louisville. Bailey's next rotation turn for the Bats also happens to be on Saturday.
"Whoever starts on Saturday needs to give us some quality innings," Baker said.
Bailey struggled during his previous one-start call-up on May 23, also against the Indians, when he gave up six earned runs on three hits with a career-high six walks over 4 1/3 innings in a 7-6 loss.
In 14 starts for Louisville, Bailey is 8-5 with a 2.71 ERA, but he has really come on lately. The 23-year-old is 4-0 with a 0.47 ERA over his past five starts. In 38 1/3 innings over that span, he's allowed just seven walks while striking out 38. With Volquez out longer than expected, this could be the best extended chance for Bailey to prove he belongs in the Major Leagues.