By Chad Finn, Boston.com Staff

So here's a telling little statistical discovery I stumbled upon this morning, one that emphasizes not only what a disappointment Jon Lester has been this season, but reminds us that for a couple of years the belief that he would and could be a true No. 1 starter was anything but a daydream.

He's no longer the active leader in the major leagues in winning percentage.

It's a distinction he has held since he qualified with his 100th career decision last year, but it's something he was on track to achieve pretty much from the advent of his big league career in 2006.

He was 11-2 after two seasons with the Red Sox, 27-8 after three, 42-16 after four ... and he entered this year at 76-34, for a damn impressive .691 winning percentage.

But with yet another mystifying, joyless and dismal performance Tuesday night -- he allowed six earned runs in four innings during a 7-5 loss to the White Sox to drop to 5-7 this season -- he backed right out of the top spot on that list.

He was surpassed by Roy Halladay, who on the same night was earning a no-decision in his first start since May 27. Lester is now at .6639 (81 wins, 41 losses), while Halladay, who is just 4-5 himself this season, is at .6644 (192-97). It's probably only a matter of time before they're both passed by the brilliant and unsung Jered Weaver (.660, 93-48 career, 11-1 this year).

(This is where I remind you that Pedro Martinez's winning percentage with the Red Sox was .760., with 117 wins and 37 losses. That's 80 games over .500 in 154 decisions. There will never be another.)

While run support and good fortune obviously are significant in a pitcher's won-lost record -- Lester's 11-2 mark in 2006-07 was accompanied by a 4.68 ERA -- his tremendous record was a fair representation of how well he pitched for the vast majority of his career.

The collective frustration regarding his current performance has led to proclamations blending anger and revisionist history. I'm as guilty as anyone, snidely tweeting last night that the Red Sox are sending the wrong lefty to the bullpen, a reference to the decision to at least temporarily remove intriguing Franklin Morales from the rotation while Lester continues to hold the season hostage with his momentum-halting performances. He's supposed to be the ace, and right now he has five wins -- as many as Daniel Bard.

We hear a lot now about how Lester was never a true ace and never will be, that he'll never be anything resembling what CC Sabathia is to the Yankees or David Price to the Rays. But the truth is that he already has been their peer -- and while maybe it feels like a long time ago right now, it wasn't.

From 2008-10, Lester ranked fourth, third and third among American League pitchers in baseball-reference.com's version of WAR. He struck out 225 batters in both 2009 and 2010. During the latter of those two seasons, he won 19 games, was fourth in the league in hits per nine (7.2) and was tops in strikeouts per nine (9.7). Some may not have recognized him as such, but he was an ace.

Why he is no longer, at age 28 and the height of his athletic prime, is mystery that puzzles even pitching experts such as Dennis Eckersley. There are the theories -- he threw too many cutters and it cost him velocity on his fastball, he melts down after calls he disagrees with, he no longer enjoys pitching in Boston (he has a 6.29 home ERA this season, 3.66 for his career).

If you're not alarmed enough by watching him contribute to making Kevin Youkilis's homecoming a wild success, consider this: Lester's adjusted ERA is 90 this season. In 1992, Matt Young had a 92 ERA+. In 1998, Steve Avery had a 94 ERA+. Now, my memory isn't as crisp as it used to be, but I do not believe those particular pitchers are remembered fondly around here.

I do believe Lester will be, and the man did build up a cachet of goodwill and great memories before last September. He's losing too many ugly decisions lately, and he's lost that statistical distinction. But it's much too soon to start thinking about Jon Lester as a lost cause.