Hideki Okajima is returning to the Red Sox in a similar fashion to the way in which he arrived. Followers of the team – and perhaps the team itself – don’t know quite what to make of the lefty.

The confusion is somewhat perplexing considering we’re talking about a pitcher who, since his unveiling in 2007, has appeared in more games than any Red Sox hurler not named Jonathan Papelbon since 2007. (Okajima has pitched in 254 games, just three fewer than Papelbon.)

During that span, Okajima has done well enough to garner somewhat of an embrace. His ERA with the Sox is 3.06, with opponents having hit .238 against him. The 35-year-old was a key element in three teams getting to the postseason, and one winning the World Series.

But, last impressions can be a powerful thing, and Okajima didn’t leave a good one. He finished an injury-plagued 2010 campaign with a 4.50 ERA in 56 appearances, leading the Sox to non-tender the arbitration-eligible reliever in November, giving him a shot at free agency.

Now, however, Okajima (who has continued to work out in the Boston area throughout the offseason) has returned, agreeing to a major-league deal with the Sox. His role (at least at the outset) most certainly won’t be what it had been the previous three years, when he was the Sox’ go-to eighth-inning option, having made 164 of his 254 big league appearances in the penultimate frame.

With Daniel Bard now established as a top setup man and Bobby Jenks signed for a similar role of getting either lefties or right-handed hitters out in the late innings, Okajima will most likely be competing with the likes of Matt Albers, Rich Hill and Andrew Miller for one of the last spots in the bullpen. That status owes to the left-handed Okajima’s diminishing effectiveness against batters from the opposite side of the plate.

WHAT HAS GONE WRONG AGAINST RIGHTIES?

There was a time -- most notably, the 2007 season -- when Okajima was effective against hitters from both sides of the plate. But that time has since passed. Here are his year by year numbers (average, OBP, slugging, OPS) against right-handed hitters:

2007: .182/.235/.277/.512, 7 extra-base hits (XBH) in 159 at-bats
2008: .234/.318/.344/.661, 8 XBH/128 ABs
2009: .309/.386/.520/.906, 12 XBH/123 ABs
2010: .340/.396/.540/.936, 11 XBH/100 ABs

Put simply, Okajima has become worse and worse in each of his four big-league seasons against right-handers. Some performance dips can be traced to bad luck; in Okajima's case, the fact that he gives up more and more extra-base hits to right-handed hitters every year suggests otherwise. By last year, any right-handed hitter who stepped in the box against Okajima represented an MVP-caliber threat.

So what happened?

Thanks to a baseball analytics program developed by TruMedia Networks (and a smattering of other resources), we were able to seek out some answers. The conclusions:

1. Okajima’s fastball velocity has diminished, and with it, so has the effectiveness of the pitch. The diminished effectiveness of his fastball.

2. He has become more dependent on his fastball against right-handers, making him ever-more vulnerable to them.

It goes well beyond the simple fact that right-handed hitters hit .340 against Okajima last year. And the fact that his fastball dropped from a “release velocity” of 87.5 mph to 86.1 mph in the past two seasons isn’t the be-all, end-all. Are they part of the problem? Yes. But they are not the isolated sources of concern.

To get an idea of how things have changed for Okajima, let’s juxtapose his ’10 season against ’08, when he was still cruising through all sorts of hitters. We’ll start with a few of the most startling numbers first:

When throwing fastballs to right-handed hitters in ’10, Okajima allowed a .388 batting average (.429 on balls put in play). Compare that to the .245/.299 clip he turned in during the ’08 season, and the picture starts to get painted.

In all, hitters totaled a .402 average when putting Okajima’s fastball in play in ’10. The number was .281 in ’08.

It should also be noted that we aren’t talking about the pitcher’s secondary weapon against right-handed hitters. Of Okajima’s 460 pitches to right-handers last season, 329 were fastballs. That’s 71 percent, a noteworthy increase from 2008, when he threw his fastball just 57 percent of the time to righties.

The reliever also led off with a heater 161 times against the 213 batters he faced. (That didn’t go well, either, with hitters managing a .467 on fastballs put in play leading off an at-bat against Okajima.)

Times have changed, and clearly so has Okajima’s already-somewhat-underwhelming fastball.

WHAT HAPPENED TO THE FASTBALL

After a serious bout of ciphering and analyzing, one conclusion can be drawn: Location means a lot to Okajima, but it isn’t the be-all, end-all.

In Okajima’s case, there does seem to be a conversion of concern when factoring in both command and velocity.

He simply doesn’t throw as hard as he used to. In ’10 he didn’t touch 90 mph a single time, while two seasons before he did it 10 times. For a pitcher like the lefty, it wouldn’t seem to be a big deal. But what should be noted is what happened when Okajima managed to crank it up to 88 mph, a number reached 107 times last season after doing it with 357 fastballs in ’08.

Against 88 mph-or-better pitches thrown by Okajima in ’10, opposing hitters managed a .455 batting average and .773 slugging percentage on balls put in play. Perhaps more notably, of the 57 high-80’s fastballs tossed to righties, 21 found the strike zone, and when that happened hitters managed a .500 BA and .875 slugging on balls put in play.

To get a better idea of how much trouble Okajima had sneaking his primary pitch past right-handers in ’10, understand that right-handed hitters totaled a .288 average on the 130 fastballs the pitcher tossed in the strike zone, compared to the paltry .198 clip managed by left-handed hitters.

In a nutshell, Okajima seemed to lose his much-needed command when trying to discover some top-shelf velocity.

WHY THERE MIGHT BE HOPE AGAINST RIGHTIES

Some of Okajima’s struggles in ’10 can be traced to physical ailments. That is undeniable. Prior to returning in September, the reliever was downright bad, allowing right-handed hitters a .368 batting average, with lefties coming in close behind at .328.

Yet, in the final month, Okajima showed something.

His fastball velocity didn’t improve (actually slightly diminishing), but the pitch’s effectiveness did.

The 14 right-handers who faced an Okajima fastball after he came off the disabled list hit just .254, with the lefty holding all righties to just a .238 average during that span (without a single extra-base hit).

While Okajima danced around the strike zone with more or less the same frequency with which he had done so in the season’s first five months, he managed to do so with greater effectiveness than the first go-round. From April-August, Okajima found the zone with 39 percent of his fastballs, with hitters making contact 90 percent of the time. After the return, batters had a contact rate of just 81 percent of the time when the lefty’s fastball found the strike zone.

It was closer to the old Okajima.

A SPECIAL AGENT?

While Okajima was worse than ever against lefties last year as well, the decline was not as devastating. Here are his year-by-year numbers against southpaws:

2007: .236/.289/.360/.648, 6 extra-base hits in 89 at-bats
2008: .184/.236/.311/.547, 7 XBH/103 ABs
2009: .167/.217/.250/.467, 6 XBH/108 ABs
2010: .284/.357/.375/.732, 4 XBH/88 ABs

Clearly, Okajima's overall numbers last year against left-handed hitters did not match his performance from 2007-09. That fact is partly attributable to the fact that he was walking more lefties than ever.

That said, it is worth noting that he was giving up fewer extra-base hits than ever to lefties (one per 22 at-bats). He allowed 21 singles in 88 at-bats by left-handers, suggesting that there was some bad luck in play. Meanwhile, his curveball was actually a tremendous left-on-left offering. Lefties hit just .118 (2-for-17) against Okajima's bender.

Moreover, in the second half of the year, lefties were largely powerless against him. He did not give up an extra-base hit to a lefty after July 6. After the All-Star break, in 37 plate appearances, lefties had a .206/.270/.206/.476 line against him -- numbers in line with his outstanding performance against lefties in 2008 and 2009. His fastball played up to lefties in the second half as well, as he garnered a .250/.280/.250/.530 line on the pitch.

ONWARD TOWARDS 2011

The reality facing Okajima is that he is unlikely ever again to be the crucial contributor that he was for the Red Sox when he burst onto the scene as an All-Star in 2007 and a key bullpen member from 2007-09.

Even so, the Sox are not signing him to be that pitcher. His opportunity now would appear to be as a complementary piece, rather than as one of the most critical two or three relievers on the team.

Instead, with the additions of Jenks and Dan Wheeler (the former of whom is a full-inning setup man, the latter a shut-down right-on-right guy), Okajima could become a pitcher whom the Sox turn to against tough lefties, and who could see further action in games when the Sox are either leading by a substantial margin or trailing. If he rediscovers some sort of solution to establish his fastball against right-handers, then he could morph into something more than that.

And if he falters? A year ago, the Sox had no real left-handed alternatives in the organization until Felix Doubront’s emergence late in the year.

This year, the Sox have plenty of alternatives with the likes of Hill, Miller and Randy Williams, not to mention Doubront.

There does still appear to be value with Okajima. The question is just how much?

Alex Speier contributed to this article.