Yoslan Herrera, whom the Pirates will officially recall from Double-A Altoona on Saturday morning, will make the spot start for Dumatrait, who was placed on the 15-day DL.
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 have the entire All-Star break to rest his left shoulder, which he says feels "achy."
"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. He was signed by the Pirates on Dec. 18, 2006, as a non-drafted free agent from Cuba. He was a member of the Youth Cuban national team for two years (1999-2000) and spent the next four seasons playing for the Cuban national team.
In July 2005, he defected from Cuba, and spent seven months in Miami. He was 3-5 with a 3.43 ERA in 10 starts this season for Altoona.
"He's a strike thrower," Huntington said. "He has a solid three-pitch mix. He's not afraid. The expectation is he'll come up here tomorrow and hopefully attack the bottom half of the zone."
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.
The Pirates also optioned pitcher Luis Munoz to Double-A to help make room for Osoria. Munoz did not pitch in his two days with the Pirates.
Todd Krise is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.