Facing his good friend and former teammate in Chris Sale -- who also dealt with trade rumors before being traded to Boston in a blockbuster deal during the offseason -- Quintana allowed seven runs on 10 hits over a season-low 2 2/3 innings in a 13-7 loss to the Red Sox.
"Tonight, for me, was a little embarrassing with my teammates. They made a lot of effort to win this game, and I can't do [anything]," Quintana said.
The seven runs were the second most he's allowed this season, only topped by the eight he allowed in his last start vs. the D-backs. The 15 runs he's given up over his last two outings are the most he's allowed in any two-start stretch of his career. Quintana's ERA on the season is now 5.60.
"It's the same thing right now," White Sox manager Rick Renteria said of Quintana's struggles. "He's not hitting his spots. Everybody's still working on finding out what it is that's leading him to leave pitches out over the plate a little bit more than anybody's accustomed to."
Quintana has been as reliable as starting pitchers come since breaking through to the Majors in 2012. He's never had an ERA higher than his rookie total of 3.76 and has worked at least 200 innings in every season since 2013.
He's faced his share of struggles since last September, when a 5.30 ERA in his final six starts of the season was different from the norm, but it didn't sound any alarms moving forward. Since the start of last September, however, Quintana has posted a 5.96 ERA in 102 2/3 innings.
The issue isn't with Quintana's stuff or his velocity. The 28-year-old's fastball velocity entering Tuesday's start was 92.5 mph, consistent with his averages in past seasons. White Sox catcher Kevan Smith said the issue isn't with the quality of Quintana's pitches. The problem is where they're located.
"I told him, when he's successful he lives in that zone, knee-to-shins, [he is good]," Smith said. "We're just a foot or two, or six inches above that. And then you saw when he really starts thinking about it, then we start losing control and missing almost too far down.
"It's going to take a game where he just believes in all of his stuff. He gets his command back and basically gets that confidence."
The White Sox, largely counted out before the season began in what was viewed as a rebuild, have overachieved during the season's two months. While the talk has not shifted from key veterans such as Quintana, Todd Frazier and David Robertson as trade targets, what they need more than anything right now is to have their ace perform to his usual standards.
They do not need Quintana, the trade chip. They need the reliable southpaw they've come to know.
"It's one of those things where it's not the stuff, because the stuff is the same, but it's location and execution," Renteria said. "If we can get that back on track, then Q is going to be what Q is, which is a very effective Major League baseball pitcher."
Fabian Ardaya is a reporter for MLB.com based in Chicago.