The Giants didn't need Peavy, who's slated to start Tuesday's series opener at Boston, to relieve Cueto. However, the mere possibility Cueto might require an early replacement demonstrated that the All-Star right-hander was not at full strength.
"That's why we wanted to cover him," Giants manager Bruce Bochy said.
Cueto remained bothered by the virus that affected him here at Tuesday's All-Star Game, when he allowed two home runs.
"I'm hoping that it goes away," he said through his interpreter, Erwin Higueros. "I have to keep working and exercising and hopefully it goes out of my system."
Cueto also felt a pain in his left side upon arising Sunday morning.
"I worked through it," Cueto said. Asked if his pain could be a symptom of a dreaded oblique injury, he replied, "I couldn't tell you. I don't know what it is. I just felt uncomfortable."
That was evident as Cueto worked five-plus innings, matching a season low, and allowed four runs and six hits. He yielded two more home runs, as Matt Kemp and Yangervis Solarte connected off him. Cueto (13-2), who absorbed his first loss since April 21, won 10 consecutive decisions in a span of 14 starts from April 26-July 6.
Cueto showed signs of his usual form, striking out four and stranding runners on second and third in his final full inning, the fifth.
"I thought he threw the ball pretty well," Giants catcher Buster Posey said. "I didn't notice his stuff being short."
Cueto indicated his command was less than precise.
"If you leave the pitches up, you're going to pay the price," he said. "That's what you have to work on. You have to make your pitches. But in the game, you're not a robot. You can't program yourself and tell yourself, 'This is what I want to have happen.' You're going to make misiakes but eventually you have to get through them."