But the right-hander's mea culpa was just beginning, soon after the Brewers had finished beating the Pirates, 9-3, in Miller Park.
"Obviously, if I had known this would happen, I wouldn't have taken the ball," said Karstens, whose official departure this time was attributed to "strained right hip flexor," as opposed to the strained right groin muscle that had ousted him last Saturday. "No one expects the starter to go out there and last a third of an inning.
"I feel bad. Flat-out, it's as blunt as I can be. It's a crappy feeling. But there's nothing I can do about it now."
The four runs he surrendered before departing were all the jump-start Brewers right-hander Mark Rogers needed.
The result did not hurt the Pirates in the Wild Card chase -- two other teams in the hunt, Atlanta and St. Louis, also lost -- although, obviously, it would have been nice to take advantage of those openings.
Pirates manager Clint Hurdle said had there been any pregame doubt about Karstens' condition "we would've not started him."
"Our training staff did its testing," Hurdle said. "He threw his bullpen and got ready at a high intensity. But you can't simulate game-speed."
Karstens' between-starts routine was positive. "I felt normal all week," he said.
He was on a smooth track right through his pre-game warmups. "He had a great bullpen, throwing everything in the zone," said Michael McKenry, his catcher.
He was hale right until the fifth batter of the game, even though the first four had already damaged him on the scoreboard.
"It wasn't until the sequence to Hart that we noticed something," Hurdle said. "And after he came out, he confirmed that something had grabbed him. He tried to get a ball away, and wasn't able to finish. And threw the next fastball at about 85-mph."
"I felt it on that [first-pitch] slider to Hart and, after that, I couldn't really get on top of any pitch," Karstens said. "I gave 'em four runs and we're supposed to try to come back from that?"
Kyle McPherson, fresh from Triple-A Indianapolis, replaced Karstens and the score reached 5-0 when McKenry let one of his pitches through for a passed ball with the bases loaded in the second.
Kevin Correia took over in the third and kept the game manageable by throwing four shutout innings.
"Great job by Kevin," Hurdle said. "He restored order and we're right in it, until the game got away from us in the eighth."
Ramirez added a majestic two-run homer that inning off Hisanori Takahashi, who was also torched for two other runs on Carlos Gomez's RBI double and Jean Segura's run-scoring single.
Ramirez thus wrapped up a 3-for-4 game with four RBIs. He has also had a nice week against the Pirates: 9-for-16, with two homers and nine RBIs.
This began as an intriguing duel between pitchers facing each other after having shut out Friday night's opposition in their most recent starts. Karstens had done it to the Brewers last Saturday, and Rogers got his turn against the Pirates on Sunday.
Karstens couldn't hold up his end of it, but Rogers treated the Bucs to a deja vu experience.
Hurdle cobbled together a lineup quite different from the one Rogers had blanked on three hits for five innings of a 7-0 Milwaukee win. Absent, for various reasons, were Andrew McCutchen, Neil Walker, Rod Barajas and Jeff Clement, who had combined to go 1-for-15, with all of Rogers' five strikeouts, in that Aug. 26 meeting.
Their replacements were Alex Presley, Jose Tabata, Clint Barmes and McKenry-- who went 3-for-15 with five strikeouts.
Interestingly, the Pirates duplicated the performance that had so annoyed Hurdle in their first run-in with Rogers, when they couldn't break through despite forcing him to throw 101 pitches in five innings.
On Friday night, the Pirates put Rogers through 107 pitches in 5 1/3, and still got to him only marginally. Their 1-for-8 showing with men in scoring position helps explain that.
"Yes, very similar," Hurdle said. "We did a lot of chasing again, and took some fastballs that were hittable. We've got to do a better job on offense. The first inning kinda set the tone -- soft for us, loud for them. We got to a bad place in a hurry."
Garrett Jones ended Rogers' string of zeroes against the Bucs at with a solo homer in the fourth, No. 22 of the season, setting a new career high after he'd also hit 21 in both 2009 and 2010.
"His stuff is always good. Location, definitely, is what he is going to battle," Milwaukee manager Ron Roenicke said of his pitcher. "His stuff is really good. I mean, there's not many starting pitchers who have that. Especially the explosive fastball that he has."
After Rogers' departure, the Bucs added a seventh-inning run as Gaby Sanchez delivered a one-out pinch-hit double against Kameron Loe and worked his way home on a wild pitch and Presley's infield grounder.