An outstanding first half by lefty Hideki Okajima enabled the Sox to open a big lead.

AP / Gregory Smith

When the Red Sox signed Japanese left-handed reliever Hideki Okajima as a free agent over the winter, his addition to the roster was viewed as Boston’s attempt to provide more heralded recently acquired Japanese pitcher Daisuke Matsuzaka with a cultural companion.

Not that the Sox were saying that, mind you. Boston’s official explanation for signing Okajima, a 31-year-old with 12 years of Nippon Professional Baseball League experience, was that he would be a plus for the team’s bullpen.

No one, though, could honestly predict what Okajima meant to the Red Sox, especially in the first half of the season.

Okajima developed a changeup to go along with a so-so fastball (88 mph) and a decent splitter. And that pitch, plus the unfamiliarity the major leaguers had with Okajima, helped him ring up a sparkling 0.83 earned-run average over the first half of the season. He was 2-0 with four saves (and one blown save) at the break.

His changeup was so effective, Okajima wasn’t a lefty specialist, giving manager Terry Francona the option of keeping him in games longer than your basic left-hander out of the bullpen.