Boston Red Sox closer Koji Uehara was experiencing lower back pain last August, the Boston Herald reported Monday on its website.

Uehara, who turns 40 on April 3, would not go into detail but said his problems at the end of last season, when he began leaving pitches up in the strike zone, had nothing to do with fatigue.

Red Sox general manager Ben Cherington revealed it was a lower back issue that forced him to change his mechanics.

"I didn't talk about it at that time," Uehara said through translator C.J. Matsumoto after arriving at the team's training camp in Fort Myers, Florida. "I think I'm over it. I'm not going to go into specifics, but it wasn't fatigue."

In six appearances from Aug. 16 to Sept. 4 last season, Uehara surrendered four homers and 10 runs. Afterward, the Red Sox removed the right-hander from the closer's role and gave him a week off. He was used in only three of the team's last 15 games.