BOSTON — He is, as the old saying goes, an enigma wrapped in a riddle inside a mystery.

Nearly two years into his Red Sox career, there’s much we don’t know about Daisuke Matsuzaka. So perhaps it’s fitting that when he takes the mound every five days or so, it’s impossible to know what to expect.

Entering last night’s start against the Tampa Bay Rays, Matsuzaka had the second-best winning percentage in the American League and among starters, had the third-lowest earned-run average. Opposing hitters were batting just .210, the lowest figure for any A.L. starter and only three starters could claim more victories.

And yet, something is amiss. Despite his 16 wins, he was averaging less than six innings per outing before last night, thanks to his absurdly high-pitch count and general inefficiency.

At times, Matsuzaka seems to succeed in spite of himself. He began last night second in the American League in walks. Improbably, in six of his wins, he walked at least five batters, the highest such total in the majors since Bobby Witt in 1987 and the most by a Red Sox starter since Mickey McDermott in 1953.

For Matsuzaka, there’s seldom such a thing as a clean outing. Last night, he didn’t have a single 1-2-3 innings. The closest he came was in the first when he faced only four hitters.

After allowing a leadoff single to Akinori Iwamura to start the sixth, Matsuzaka was through, needing 102 pitches to record 15 outs. In a game in which the Sox had the opportunity to leapfrog into first place for the first time since just after the All-Star break, Matsuzaka forced the bullpen into taking care of the final 12 outs.

Jason Bay’s two-run homer in the eighth put the Sox three outs away from a comeback win, but Jonathan Papelbon imploded in the top of the ninth, handing the Rays back the lead in the span of four batters for a 5-4 Tampa win.

Last night marked the eighth straight start in which Matsuzaka had thrown more than 100 pitches, and yet, only three times in that span did the pitcher get past the sixth inning.

Before last night’s game, manager Terry Francona was asked if he ever allowed himself to think about just how dominant Matsuzaka could be if he could only conquer his command issues.

“Be careful what you wish for,” cautioned Francona. “If a guy all of a sudden sacrifices what he’s doing … This guy’s been pretty good. The bar was set so unfairly high when he came over here that whatever he does is going to get picked part. He’s given us 31 wins (in two seasons) — that’s a lot of wins. And he seems to be understanding and getting better as he goes. So I think we’ll take that.”

What’s maddening for the Sox, however, is that Matsuzaka doesn’t seem to trust his stuff, which, if not considered “plus,” is at least above-average. When he throws strikes, hitters seem incapable of much damage. He’s allowed only10 homers this season in 26 starts. Moreover, only 16.1 percent of his pitches before last night were put in play, the fourth lowest figure in the A.L.

Thirteen times this season, Matsuzaka has allowed four hits or fewer — a measure of how dominant he can be. But the low hit totals are negated by his penchant for walks. In fact, they’re more than negated, since walks produce base runners and do so while ratcheting up the pitch count.

Even when he gets himself into trouble, he’s fully capable of extricating himself. With runners in scoring position, opposing hitters were batting a feeble .190, the lowest average for any A.L. pitcher.

Even with the no-decision last night, Matsuzaka has already topped his win total from his rookie season with three more starts to follow.

But Matsuzaka’s been eclipsed in the rotation. Jon Lester will be the team’s No. 2 starter in the postseason with Matsuzaka the third starter.

It should help Matsuzaka’s cause in October that he will not have to face patient, grinding teams. Both potential first-round matchups — the White Sox and Angels — are notoriously aggressive. Then again, that didn’t help Matsuzaka much last postseason; he couldn’t get through five innings against the Angels in Game Two of the ALDS, earning a no-decision.

If you’re the Red Sox, it’s a sign of a deep rotation that your third starter will end up with a minimum of 16 wins. But it’s also a frustrating reminder that Matsuzaka could — indeed — be better.

smcadam@projo.com