Manager Bruce Bochy said after the game that Moronta suffered an acute shoulder strain.
Moronta fell off the mound and writhed on the grass after throwing a 98-mph fastball to Luis Urias on his third pitch. The reliever delivered a swinging strike, but recoiled with a yell before dropping to the ground. He held his right arm gingerly as he walked off with the help of a trainer.
The 26-year-old came into the game in relief of rookie Logan Webb with two outs in the sixth inning.
"There's no question there's concern. We have to wait for the [MRI] results though,'' Bochy said. "It's a tough break for Reyes. He's one of our guys and I'm sure he's going to be down for a while.''
Moronta was 3-7 with a 2.86 ERA in 55 relief appearances entering Saturday. The second-year player has 70 strikeouts in 56⅔ innings.
Information from The Associated Press was used in this report.