As they prepared for the 2011 season, most observers considered the Red Sox a strong club that possessed an obvious flaw. The additions of left-handed hitters Adrian Gonzalez and Carl Crawford – and the corresponding departures of switch-hitter Victor Martinez and the right-handed Adrian Beltre – would create a sort of kryptonite.

In a division that is loaded with several of the most outstanding left-handed pitchers in the game, the Sox would be vulnerable to southpaws. At least, that was the prevailing line of thinking prior to the start of the season.

Some suggested that those concerns about a deep Boston club were somewhat overblown. But no one anticipated what has unfolded to this stage of the season.

On Friday night in Toronto, the Sox claimed a 5-1 win over left-hander Jo-Jo Reyes and the Blue Jays. (Recap.) Amazingly, it was the 10th straight win for the Sox when the opposing starter has been left-handed. It has now been almost a month and a half since Jason Vargas of the Mariners claimed a victory over Boston; since then, the Sox have averaged more than six runs per game when the opponent has had a southpaw on the hill.

It is the longest Sox winning streak against left-handed starters since the beginning of the 1998 season, when the team reeled off 13 straight victories in games when they faced a southpaw starter. The Sox’ 14-7 record against lefties this year is the second-best in the AL, behind only the Tigers’ 14-6 mark; the team's .667 winning percentage in such contests is also a marked improvement on its 23-19 record (.547) when a right-hander starts. 

It is a remarkable showing by a team that features four left-handed hitters (Jacoby Ellsbury, Adrian Gonzalez, David Ortiz, Carl Crawford) in most contests with a southpaw on the hill. Moreover, switch-hitting catcher Jarrod Saltalamacchia has struggled against lefties in his career.

Despite the fact that the Sox skew left, left-handed starters are 6-11 with a 5.11 ERA against the Sox. After Friday night, the team is hitting .269 against southpaws (4th in the AL; 6th in the majors) with a .344 OBP (5th; 7th) and .430 slugging mark (4th; 5th). Those marks are extremely close to what the team has done against right-handers, against whom it is hitting .273 (1st; 2nd) with a .343 OBP (1st; 2nd) and .444 slugging percentage (1st; 1st).

Some of the team’s production against lefties has come from expected contributors. Jed Lowrie’s .397 average against lefties is the best in the American League (min. 50 at-bats). Dustin Pedroia, despite his struggles this year, has continued to serve as a southpaw slayer. He is hitting .369 with an amazing .506 OBP (best in the majors), along with a .523 slugging percentage, against lefties. Kevin Youkilis has a .281 average and .985 OPS against them.

But the team has also enjoyed contributions from those who were expected to be most vulnerable to southpaws. Most notably, David Ortiz has been an equal-opportunity force this year, performing at an elite level regardless of the handedness of his opponent.

The DH is hitting .333 with a .425 OBP and .551 slugging mark against left-handers, in the process reasserting himself as an everyday lineup staple and transforming the middle of the Sox order. With Jacoby Ellsbury (.282 average, .342 OBP, .394 slugging) and Gonzalez (.287, .344, .356) remaining productive against lefties, the Sox have featured a lineup that has afforded portsiders little room to breathe.

The first six members of the Sox lineup against lefties – Ellsbury, Pedroia, Gonzalez, Youkilis, Ortiz, Lowrie – has dominated that subset of pitchers for a combined .323 average, .404 OBP and .500 slugging percentage. The combined production of that group has been too much for opposing pitchers to navigate, explaining the team’s longest winning streak against lefties since Pedro Martinez’ first days in Boston.

That top six also helps to explain how the Sox have already become the first team since 2006 to pin three losses on Yankees left-hander CC Sabathia since 2006, and how the club is amidst a run against lefties that would have seemed almost impossible entering the year.

The run, of course, will come to an end. It is worth noting, after all, that aside from two victories against Sabathia and one in a game started by A's standout Brett Anderson (just before he landed on the DL with an undiagnosed injury that has him seeking opinions from Dr. James Andrews), the Sox haven’t exactly been making hay against Steve Carlton.

Instead, the Sox have been methodically racking up victories against the likes of Brian Duensing and Josh Outman, Andy Oliver and Phil Coke, Doug Davis and Jo-Jo Reyes.

Nonetheless, the Sox are giving evidence that a perceived vulnerability entering the season was, at the least, dramatically overstated. As they have steamrolled their way to the best record in the American League, the Sox have shown that there is no simple, single formula – aside from health – that can derail their surge.