The Boston Red Sox, demonstrating confidence that Koji Uehara's late-season woes were an aberration and not a harbinger of imminent decline, signed the 39-year-old closer to a two-year, $18 million deal, the team announced Thursday.

The Red Sox have signed closer Koji Uehara to a two-year contract, the team announced Thursday. Justin K. Aller/Getty Images

The Red Sox signed Uehara before he became eligible to negotiate with other clubs Tuesday. His deal nearly doubles the two-year, $9.25 million contract he signed with the Red Sox before the 2013 season, before he had been identified as the club's closer. It is $2 million less than last's winter's two-year, $20 million deal the Detroit Tigers gave to free agent Joe Nathan, who is perhaps the closest comparison to Uehara, given that he was 38, a year younger than the Japanese right-hander.

Nathan was coming off an All-Star season with Texas in 2013 in which he saved 43 games and posted a 1.39 ERA, striking out 73 while walking 22. But even though he saved 35 games in 2014, he was a major disappointment for the Tigers; his 4.81 ERA was the second highest of his career, his strikeouts per nine innings ratio (9.84) was the lowest of his carer and his rate of walks per nine (4.5) was a career high.

Uehara, who became the Red Sox's closer in mid-June of 2013 after injuries to Joel Hanrahan and Andrew Bailey, was spectacular in the role, carrying that performance through a postseason in which he converted all seven of his save opportunities while posting an 0.66 ERA.

"You're really looking at a guy who has been one of the elite relievers in baseball for several years," Red Sox general manager Ben Cherington said. "And again, based on our evaluation at the end of the year and what he did, we did not see any reason why that can't continue.''

In 2013, he led all major league relievers (40 innings minimum) in ERA (1.09), opponents' batting average (.130), WHIP (0.57), and strikeout to walk ratio (11.22). His WHIP was the lowest of any pitcher in big league history (40 innings or more).

With only minor slippage, Uehara spent much of 2014 duplicating his historic performance of the year before, converting 26 of his first 28 save opportunities while posting a 1.27 ERA and .182 opponents' batting average while striking out 70 and walking just seven. That all changed starting Aug. 15, when Uehara made the first of seven appearances in which he was roughed up for 10 earned runs in just 5 2/3 innings, giving up four home runs while posting a 15.88 ERA. After giving up two home runs in the bottom of the ninth inning of a walk-off loss to the Yankees on Sept. 4, Uehara asked to be removed from the closer's role.

"We identified some things that led to his brief period of struggles,'' Cherington said, "and he came back and threw well in his last three outings."

Uehara threw three innings over the course of the team's last 22 games, none in a closing situation, which would hardly seem to be enough to declare all was well. But evidently the Sox believed otherwise.

"From a health standpoint, he had a little lower back issue at the end of the year, but that was resolved and wasn't really a concern going forward," Cherington said, making the first mention by any Red Sox official that the pitcher's back had been bothering him.