The greatest unknown in the home clubhouse at Fenway Park is 6-foot-1-inch and roughly 195 pounds, and, naturally, is lefthanded. Off the field, as on it, Hideki Okajima comes and goes so quickly sometimes that no one seems to notice.

On Tuesday, after the Red Sox clinched their fifth playoff appearance in six years with a 5-4 victory over the Cleveland Indians, one of the more striking aspects of the Sox’ postgame celebration was that Okajima was not there. Not really. The man who all but punched Boston’s ticket to October – he recorded the biggest out of the game – briefly celebrated with his teammates on the field after the game. Okajima then showered, dressed, and scurried out of a home clubhouse where the large majority of Sox players shared in their latest collective achievement.

Yet, when the Red Sox begin playing meaningful games again, nobody may mean more to their success than their international man of mystery.

"I think people have been too critical of him for not repeating last year, which was impossible because he set such a high standard," Red Sox general manager Theo Epstein said of Okajima, whose numbers are actually quite consistent with those he posted in 2007. "He’s been a valuable relief pitcher for the organization."

As we are wont to do, let’s take a moment here to reflect. Working (and living) in a foreign land is a difficult challenge for anyone, and for some more than others. The tumultuous Byung-Hyun Kim years taught us that. Okajima has had the additional burden of dealing with the presence of $103-million-man Daisuke Matsuzaka, who is 18-2 with a 2.80 ERA this season, and 33-14 with a 3.69 ERA in his major league career. Members of the Boston organization recently described Okajima as "aloof" and "sensitive," and one suggested that he does not have much contact with anyone outside of his translator, Jeff Yamaguchi.

And yet, Okajima continues to pitch quite well, all things considered. In his last 29 appearances, Okajima is 2-0 with a 2.05 ERA; during that span, he has held opponents to a .151 batting average, .255 on-base percentage and .247 slugging percentage, numbers that are superior is some aspects to those he posted last year (.202, .255, .306). Admittedly, Okajima’s biggest problems this season came with inherited runners, which means there was no real damage to his résumé when those men crossed the plate.

Still, Tuesday night, Sox manager Terry Francona summoned Okajima with two outs and the bases loaded in the seventh inning of game the Sox led, 5-4. The hitter due up was the switch-hitting Victor Martinez, who was 0 for 5 with three strikeouts (including postseason) against Okajima in his career. Okajima battled Martinez through a full count (it was an eight-pitch at-bat) before retiring Martinez on a foul pop to first. Okajima turned the ball over to Justin Masterson in the eighth.

Know why this was especially noteworthy? Last week at Tampa, with the Rays in the midst of a late-inning rally against Masterson, Francona left Okajima in the bullpen despite chances to use him against the lefthanded-hitting Cliff Floyd (then 1 for 10 against lefthanders this year) and the switch-hitting Dioner Navarro (a .309 hitter from the left side, .254 hitter from the right). Francona wanted sinkerballer Masterson in the game partly because he needed a double play – Okajima has induced just one all year – but the manager also has shied away from using Okajima with men on base during much of the second half.

Until Tuesday.

"I thought there was more to gain than lose," Francona said. "Seeing Okajima get a couple of those under his belt would be really beneficial. I don’t think there’s any way we get through the playoffs without bringing in Okajima with men on base."

Lest anyone think this provided some new, foolproof structure for the bullpen, think again. On another night, in a similar situation, the Red Sox might opt for a different matchup. The emergence of Masterson and the complexities of using Okajima mean that Francona and pitching coach John Farrell may adjust the plan on a daily basis, opponent to opponent, lineup to lineup. Some nights Okajima might get the eighth, some nights the inning may belong to Masterson. It all may depend on the score, the scheduled hitters, the situation, the schedule.

Last year, his first in the major leagues, Okajima was nothing short of brilliant before tiring in the second half. The good news this season is that he is considerably stronger at this stage of the year. For what it’s worth, Okajima has sensational numbers against the Los Angeles Angels – a 1-0 record, 1.04 ERA and 10 strikeouts in 8 2/3 innings over seven appearances – and he has handled both Garret Anderson (1 for 6) and Vladimir Guerrero (0 for 4). Torii Hunter is 2 for 5 against him; Mark Teixeira is 1 for 2.

For much of this season, unlike Matsuzaka, Okajima has boycotted the Japanese media, who found the pitcher to be equally difficult while he was playing in Japan. Maybe that is a good sign. Okajima was a consistent and reliable pitcher overseas before coming to the Red Sox last season, and as October beckons, the Red Sox need him to be consistent and reliable now.

For their sake, let’s hope Okajima is coming and not going.