All along, there was the great unknown.

When the Red Sox paid a posting fee of more than $51 million for the rights to negotiate with Daisuke Matsuzaka, and then committed an additional $52 million over six years to the pitcher, the proclamations about his talent were far-reaching. He was one of the most talented pitchers in the world. He had a diverse array of pitches unlike anyone had ever seen. On and on the accolades went.

Yet no one could say with any certainty how the right-hander would hold up, at least based on precedent.

That was not to say the Red Sox didn’t do their research. They had studied the issue of the aging patterns of Japanese pitchers, and perhaps more importantly, they had seen the condition of Matsuzaka’s shoulder and elbow before they finalized their landmark deal with him.

“Something that's more conclusive [than studies] is getting inside a player's arm,” one team source said in 2007. “We do know what the inside of Daisuke's arm looks like.”

Matsuzaka’s workload in Japan — the legendary performance at the Koshien tournament, starts that had lasted 200 or more pitches in which the right-hander was throwing 97 mph in the ninth inning, the between-starts throwing regimens that were unfathomable to most U.S.-raised pitchers — was well known. But the Sox felt that, based on what they had seen in his arm, he was a good bet to remain healthy for much of his six-year contract.

Even so, it remained fair to wonder how many bullets were in his famous arm. Matsuzaka made his debut in the NPB (the Japanese major leagues) at age 18, when he threw 180 innings. By the time the Seibu Lions made him available to major league teams via the posting process, he’d thrown 1,402 2/3 innings against elite competition through his age 25 season.

“That,” said one executive of a team that opted not to bid for Matsuzaka in the posting process, “is a pretty good career for a lot of pitchers.”

And so, the Sox were acquiring a player in his prime, but one with a workload that does not often lend itself to durability, at least among pitchers in the U.S.

In Major League Baseball, there have been 19 pitchers since 1980 who have accumulated as many as 1,000 innings by the time they turned 25 years old. Four of them are active, making it premature to examine how heavy early usage impacted their careers (though both CC Sabathia and Jon Garland now have turned in several healthy years after turning 25).

Of the remaining 15 who are no longer pitching in the big leagues, 11 of them pitched fewer — in most cases, far fewer — innings after their age 25 season than they did to that point. The exceptions? Greg Maddux, Roger Clemens, Mark Gubicza and Bill Gullickson. Just two of those 15 (Greg Maddux, Alex Fernandez) had lower ERAs after their age 25 season than through it.

So, over the last three decades or so, the only pitcher to maintain his durability and improve his performance after managing an enormous workload through his age 25 season was Maddux, a future Hall of Famer who is arguably the greatest pitcher of the post-integration era in Major League Baseball.

When Matsuzaka arrived in the majors, it was difficult to say whether familiar aging patterns (and diminishing returns) would apply to him. His baseball upbringing had been radically different from that of American pitchers. Back in 2007, it was thought that he had simply been conditioned to handle a different workload than his U.S. counterparts and that he might follow a different aging/injury pattern.

Now, in his fifth year in the majors, it would be difficult to conclude that all of the bullets Matsuzaka used as a teenager haven’t depleted his arsenal now that he is a 30-year-old. He went 108-60 with a 2.95 ERA with 1,402 2/3 innings en route to superstardom in Japan; he is now 49-30 with a 4.25 ERA in 622 2/3 innings for the Red Sox.

On Tuesday, he was placed on the disabled list for the sixth time in the last four years. This is the fourth straight year in which some kind of arm issue has sidelined him:

— In 2008, he missed the first three weeks of June due to what the Sox described as a mild right rotator cuff sprain.

— In 2009, after getting injured in his preparation for the World Baseball Classic, he went on the DL for five weeks in April and May, then went back on the disabled list for roughly three months from June through September. Both of the DL trips were for a “mild right shoulder strain,” though the Sox felt that the pitcher needed to improve his total body conditioning in order to be able to pitch effectively.

— In 2010, he opened the year on the DL due to a neck strain and returned there in June due to a right forearm strain.

— And now, in 2011, he is on the DL with a right elbow sprain. This comes less than three weeks after he was pulled from a game with cramping in his elbow, an injury that now appears to have been a harbinger to this spell on the sidelines. Since then, his velocity has fallen steadily, and his command was so bad on Monday against the Orioles — when he walked seven — that the Sox feared that he was wounded.

Despite that injury history, it would be a mistake just to dismiss Matsuzaka as a bust.

He was very good for the Sox in 2007, when he went 15-12 with a 4.40 ERA, 204 2/3 innings and 201 strikeouts, while also winning a pair of postseason games — most notably, in Game 7 of the ALCS, when he allowed two runs in five innings. Given the alternatives on the market as free agents entering in the 2006-07 offseason (Barry Zito, Jeff Suppan, Gil Meche and Jason Schmidt — along with the one pitcher to truly earn his keep, Ted Lilly), it is entirely possible that the Sox wouldn’t have been a World Series team without Matsuzaka.

In 2008, he was the beneficiary of good fortune in going 18-3 with a 2.90 ERA despite leading the majors with 94 walks. Still, there were many starts where he verged on unhittable, and he had his moments where he was overpowering.

But since the start of the 2009 season, he is just 16-15 with a 5.03 ERA, and most notably, he’s only been available to make 44 starts and pitch 250 1/3 innings during that time.

For a two-start stretch this year, he looked like a pitcher who was positioned to make an impact on the Sox staff. In back-to-back starts, he logged seven and then eight shutout innings while allowing just one hit in both outings, one against the Blue Jays, the other against the Angels.

But in three subsequent starts, his fastball velocity steadily diminished. For the year, according to Fangraphs, his average fastball velocity of 90.6 mph has been his worst since arriving in the U.S.

Perhaps his current diminished velocity readings are just the byproduct of a specific injury. Perhaps they are reversible.

But maybe not. Maybe they are more sinister. Maybe, at an age when many pitchers are nearing or just entering their decline phases, he is well into his, thanks to a workload that made him a legend in Japan but that left him fighting against time almost as soon as he got to the United States.