Both Dumatrait and general manager Neal Huntington said the injury is not as severe as the rotator cuff tendinitis that first caused Dumatrait to go on the DL on June 22.
Dumatrait will be able to rest during the All-Star break.
"It's not as bad as it was," he said. "It is definitely frustrating because I did work hard to get back. It kind of just flared up again. Hopefully, [I'll] just need work hard again and it will go away next time.
"I just don't want to make it worse," he added. "By throwing, it irritates it more. We'll see what happens."
Saturday's start will be Herrera's Major League debut. The Bucs signed him on Dec. 18, 2006, as a non-drafted free agent from Cuba.
He was 5-8 with a 3.11 ERA in 17 starts this season for Altoona.
Reliever Franquelis Osoria will join Herrera in Pittsburgh on Saturday. The right-hander is expected to be recalled from Triple-A Indianapolis, where he spent five days rehabbing his right ankle.