NEW YORK — At one level, a look at the Red Sox rotation would suggest mediocrity.

Through 40 games, the team’s starters have a 4.12 ERA, a mark that ranks 18th among the 30 major league teams and 11th among 14 American League squads. Their 18 quality starts likewise rank in the bottom fifth of the majors.

And yet, while those numbers suggest a rotation that is unimpressive, closer investigation suggests that, through the first 40 games of the season, this Red Sox rotation has shown the potential to be the most dominant in years. That point has been illustrated by the team’s best stretches of the year, including the freshly concluded three-game sweep in the Bronx that ended with a 7-5 victory Sox over the Yankees on Sunday night (recap).

Jon Lester put the punctuating mark on the sweep, earning a victory by recovering from a rocky initial two innings — during which he allowed four runs on four hits — to overpower the Yankees. He did not give up another run while permitting just one hit over his final 4 1/3 innings, with a pitch mix during that span that was more indicative of his talents.

“He didn’t have his best stuff at the beginning. He was a little erratic, but he bounced back and threw the ball well,” Sox third baseman Kevin Youkilis said. “That’s Jon. He might have one bad inning, but he’s going to come back and dominate. It fuels him sometimes. He kept us in the ballgame and threw the ball well.”

Lester’s start was the least impressive of the weekend set for the Sox. Certainly, the left-hander was reluctant to accept any praise on a night when he improved to 5-1.

“The offense did a great job tonight of picking me up,” Lester said. “I tried to ruin another good offensive night for us, but they did a great job changing the whole momentum of that game.”

Yet considering the opponent — a Yankees team that features the sort of quick-strike offense that can knock a pitcher out of a game early and then plunder a bullpen — Lester’s recovery was significant. And it helped the Sox to claim an accomplishment of some distinction.

Lester followed Clay Buchholz (seven innings, two runs on Friday) and Josh Beckett (six shutout innings) in claiming victories over the Yankees. It was the first time since 1975 — when Reggie Cleveland, Roger Moret and Bill Lee won three straight in a late-July series in New York — that Sox starters claimed wins in three straight games of a series in the Bronx.

The Sox have shown other glimpses of a rotation that, anchored by Lester, Beckett and Buchholz, is capable of accomplishing a great deal. The team is 4-0 in games when the opponent featured a former Cy Young winner as a starter this year. And at one point at the beginning of the month, the team outpitched Felix Hernandez, Jered Weaver and Dan Haren in three straight contests.

In stretches, at least, the Sox have shown the ability to hold their own against the league’s elite, offering a glimpse of a rotation that has the ability to succeed against both top offenses and top opposing pitchers.

For several years, the Sox have tried to assemble a rotation that would feature a wealth of arms that rank among the best in the game. But for the most part, the team rarely has had more than two pitchers who were performing to top-of-the-rotation standards, with others — John Smoltz, Brad Penny, John Lackey — failing to live up to that billing.

In that regard, the early returns of the 2011 rotation have been intriguing. Right now, Beckett leads the AL with a 1.75 ERA, while Lester has a 3.28 mark and Buchholz is sitting at 3.94. Just once since 1986 have the Sox featured three starters who finished the year with sub-4.00 ERAs (2002, when Pedro Martinez, Derek Lowe and Tim Wakefield did so).

The re-emergence of Beckett thus far this year has been eye-opening. It is early in the season, but it would be hard not to suggest on the basis of his eight starts this year that he has once again recaptured the dominance that characterized his All-Star campaigns of 2007 and 2009.

Indeed, after extending his scoreless innings streak to 18 1/3 innings, the now-31-year-old (an age he reached on Sunday) is off to a start that rivals any in his career. Though he has a 3-1 record that reflects the absence of run support, his 1.75 ERA offers an indication of his steady dominance, as does the fact that he hasn’t given up a run in half of his starts this season. Even in 2007, when he was 7-0 through his first eight starts, Beckett had a more modest 2.66 ERA.

Again, it’s necessary to offer the caveat that it’s early — very, very early. Even so, the glimpse offered by the Sox to this stage of the year hints at the possibility that this could be one of the finest Sox rotations in decades.

Before diving further into that notion, some other caveats are necessary. First, the fact of the earliness of the season means that things can change dramatically. In particular, once the weather warms and balls start to fly out of parks, the perspective on any pitching staff can change. Health could change the outlook, and for that matter, so could the ebbs and flows of pitchers’ feel for their craft.

Moreover, it’s important to consider context when looking at the Red Sox pitching staff. Specifically, one needs note that offense across the majors has tanked this year. Entering Sunday, teams were averaging 4.20 runs per contest, significantly down from the big league mean of 4.38 scores a game last year, which was itself far and away the lowest level since clubs averaged 4.12 runs a game in 1992.

All of that said, the Sox have the makings of a dominant group of starters unlike anything that they’ve been able to feature in years and years.

To date, Beckett features top-of-the-rotation stuff. Most notably, he’s been able to mix two- and four-seam fastballs with a changeup that has become a devastating weapon. He’s also featured a terrific swing-and-miss curveball in most of his outings, and when that element was missing on Saturday night, he had his best cutter of the year.

Asked whether Beckett has been as good this year as he was in the best stretches of his career, catcher Jason Varitek pulled no punches.

“I see him in some aspects better,” Varitek said. “Josh still has power and deception. … His changeup’s come a long way. We’re incorporating it a lot more, making him a little more complete.”

Buchholz now appears to have emerged from his slow start. He has dropped his ERA to 3.94, and since the beginning of 2011, his 2.67 ERA is second only to the 2.49 mark of Felix Hernandez among American Leaguers.

In his most recent two outings, he has shown the full complement of pitches — two-seamer, four-seamer with velocity, curveball, changeup, cutter — that makes him an immensely frustrating opponent.

“Clay’s got every pitch as a plus-pitch,” Varitek marveled.

Lester, meanwhile, entered Sunday with a 3.19 ERA that ranks seventh among AL starters since the beginning of 2010. He has cemented his credentials as one of the best and most reliable starters in the game.

This has quickly emerged as a rotation with a total capacity for shutting down opponents. Already, with less than a quarter of the season complete, the Sox have had 10 games in which their starter has not given up a run. In 39 games, the team has matched the number of scoreless starts that the 2006 Sox received in 162 contests.

Halfway through May, the Sox are nearing the number of shutout starts that they received from their playoff clubs of 2003 (16), 2004 (17), 2005 (13) and 2007 (15). In fact, the team is on pace to shatter the standard set by the 2002 club, which had 30 starts without yielding a single run — the most such starts in any season by a Sox club since at least 1919, according to baseball-reference.com.

In fact, in that 82-year span, the most scoreless starts by any team was the 34 turned in by the 1964 Los Angeles Angels. The Sox are pitching at a pace that could yield 42 such starts.

They’re unlikely to sustain that ridiculous pace, and it is important not to exaggerate the overall performance of the Sox pitching staff. The team ERA is 4.27, a mark that ranks 12th in the AL.

That said, the poor overall ranking reflects a handful of bad outings. The Sox have gotten five catastrophic starts (second most in the majors, behind the Twins) that ended with a yield of at least seven runs (three from Lackey, one from Daisuke Matsuzaka, one from Tim Wakefield). Those have distorted the team’s overall ERA but should not diminish the reality that the Sox have enjoyed as many — if not more — dominant starts than any other staff in baseball to date.

“There’s only a couple of games that made those numbers not near what they could be,” Buchholz said. “I think things are running pretty smoothly right now. Guys are, for the most part, guys are going out there, throwing six, seven good innings, giving the team a chance to win.”

All of that could change quickly. Any of the three could get injured or suffer from a performance slide that represents a normal ebb to the season.

Even so, to this point in 2011, the Sox have featured three starters with the talent to match up with any team in the American league.

“They all have the ability to be equally dominant,” reliever Daniel Bard said. “We were kind of one quality starter from being really good last year — and a few bats, because everyone was hurt. But it’s big. And the other two guys, [Lackey] and [Matsuzaka], have had flashes of being really good this year, too. It’s just a matter of consistency. The ability is there. The sky’s the limit if we can get those guys on track where they have the ability to be.”