Uehara has given up five home runs, all of them since May 17. He surrendered only three all of last season.
"I can't really tell," Uehara said through an interpreter. "I'm working on it myself."
Just as the finish on Uehara's splitter once made him dominant, the lack of finish on it of late has made him ordinary.
"At times it's the action to the split," said manager John Farrell. "It hasn't had that late tumble. He's had it on occasions. Tonight was a night where he comes in with ample rest. It's been the late action to the split that's been the difference on a consistent basis versus years past."
Uehara has a 4.78 ERA in 28 outings. Over his last 12 appearances, his ERA is 7.36.
Junichi Tazawa, Matt Barnes and Heath Hembree are the candidates to get work in the eighth inning if Farrell decides to move Uehara from that spot.
"Right now, obviously, after a tough night for Koji, I respect and understand your question, but we've got to take a look at the internal guys," said Farrell. "We have to see who might be more ready for certain matchups. I'm not here to say that Koji's out of the setup role."
Ian Browne has covered the Red Sox for MLB.com since 2002. Follow him on Twitter @IanMBrowne and Facebook. This story was not subject to the approval of Major League Baseball or its clubs.