The Sox gave him a 4 to 3 lead.
He goes out for the 7th, leaves a ball up, and gives up a line-drive single to a lefty hitting .160 or something. That should not happen. He's supposed to be a big-time pitcher. Pitchers with their wits about them make certain that they don't get careless in that situation. They go for the kill. They put that kind of batter away -- the first batter after your club takes the lead. That's one of the ways big-time pitchers are defined. The minute that happens, the club in the lead starts to lose the grip it had on the other team. You give the team that was down new life. Heck, they get a banjo hitter on right away, anything can happen -- and it did. 
The error earlier hurt Lester, no doubt. But a pitcher hurt by an error is not a rarity. What counted in that game was Lester's inability to close the sale. If he goes seven innings and leaves with a 4 to 3, it's a different game. Maybe the pen still blows it. Maybe not the way it has been performing lately. It would have got a clean slate in the 8th.