Diamondbacks starter, he compiled a record of 1–5 with a 3.56 ERA. On May 29, Kim was traded to Boston for Shea Hillenbrand. Kim remained as a starter through June, but the Red Sox needed him as their closer because the Red Sox's closer-by-committee approach implemented following the advice of statistician and Red Sox adviser Bill James was failing. On June 27, Kim made his final start for the 2003 season in Boston's 25–8 win against the Florida Marlins at Fenway Park. Kim became the Red Sox closer in July, converting 16 out of 19 save opportunities despite pitching through lingering ankle and shoulder pain caused by the injury he sustained in April. Although he did not give up an earned run in September, by the time the postseason started Kim was not healthy enough to be effective on the pitching mound. Kim was pulled in the bottom of the ninth inning with two outs in his only ALDS appearance in Game 1 against the Oakland Athletics. During the lineup announcement in Game 3 at Fenway Park, Red Sox fans intensely booed him. Thinking that he did not deserve the booing after he pitched despite being in pain to advance the Red Sox to the postseason, Kim gave them the middle finger, but later issued an apology. Because of shoulder stiffness, Kim was left off the ALCS roster. As a starter, Kim went 3–6 with a 3.38 ERA in 12 appearances in . In the beginning of 2004, however, problems with his balance—a lingering effect of the 2003 ankle injury—made him ineffective and cost him a spot in the starting rotation after going 1–1 with a 6.17 ERA in three starts. Bronson Arroyo took Kim's starting rotation spot and Kim was optioned to Triple-A Pawtucket in May. Kim rejoined the Red Sox in September. He was assigned to the bullpen and won one game in 5⅔ innings of work. Despite not being on the Red Sox roster for their 2004 World Series championship run, he was awarded his second World Series ring by the team. Kim briefly became a subject of talks between high-ranking U.S. and South Korean diplomats in August 2004. Upon his arrival in Seoul as the new U.S. Ambassador to Korea in August 2004, Christopher R. Hill spoke with his Korean diplomatic counterparts about Kim and the Red Sox, along with the U.S.–Korea relationship and other diplomatic