Penny said he felt discomfort prior to his appearance during Monday's game, as well during his two-inning outing in which he allowed six runs in a 14-2 loss to the Nationals.
Otero had made the Giants' Opening Day roster but was optioned to Triple-A on April 27 to make room for Eric Hacker, who was making a spot start for the Giants at that time. Otero rejoins the Giants after going 4-4 with a 2.83 ERA out of Fresno's bullpen.
Penny had experiencd shoulder problems before joining the team on June 29, but both Penny and manager Bruce Bochy said they were unsure whether the shoulder problems are connected.
The longtime starter and two-time All-Star Penny has struggled in his transition into a reliever, holding a 6.86 ERA in 21 innings through 15 appearances for the Giants.
"We just want to give him time to get this cleared up and get right," Bochy said.