Uehara said after the game that he felt something before the strikeout pitch to Belt.
"It's some muscle soreness that I feel," Uehara said through a translator. "I'm just praying that it's not that serious."
Uehara's injury has the potential to be a big blow to the team's bullpen. With Craig Kimbrel on the disabled list with a left knee injury, Uehara has been acting as the team's closer, converting his last five save opportunities.
Right-hander Junichi Tazawa, who's on the disabled list with a right shoulder injury, is expected to return to the bullpen as early as Friday.
Before Tuesday's game against the Giants, Farrell mentioned that right-hander Joe Kelly, who's been dominant with Triple-A Pawtucket since his demotion June 1, will move to the bullpen upon his return. That move could be expedited depending on the severity of Uehara's injury.
Right-hander Brad Ziegler, who was recently acquired from the D-backs, had 18 saves while with Arizona.
"As we've had to go to other guys to step up, we'll look to do that. I can't tell you at this point how much time Koji will miss," Farrell said. "Taz is soon to return to us, to give us a another quality arm that's a proven late-inning guy. With Brad here, again not knowing how long Koji will be down, we've got other options to go to to close out ballgames."