The right-hander was scratched from his scheduled turn to throw batting practice after he complained of "a little soreness" in front of his upper shoulder, in "the biceps area."
For anybody else, the development could have been easily shaken off. It is early. The past couple of mornings have been unseasonably chilly, not ideal throwing weather.
But this, after all, is Karstens. He could extract only 90 2/3 innings out of his body last season. That fragility, in fact, is the very reason the Pirates at first non-tendered him, then delayed bringing him back on a new contract until Jan. 17.
So this is not the first arm the Bucs wanted to provide an "uh-oh" moment, only days after Karstens had proclaimed he felt good because he had started conditioning earlier.
"There's no real concern," said general manager Neal Huntington. "He just had a little soreness in the biceps area. We'll just change his program a little bit, and see where he is in a couple of days."
The 30-year-old was in a good mood as he briefly discussed the hitch, making sure to emphasize he wasn't exactly entering shut-down mode.
"I just want to play it smart. It doesn't make any sense to push it," Karstens said. "I'll still play catch [on flat ground]. We just want to be smart about this, wait a couple days. It's not September. I don't want another Arizona."
Karstens was referring to his start last April 17 against the D-backs, which he had to leave after one inning with tightness in the shoulder. Including a strained-groin glitch in his rehab program, he didn't resurface until late June.
This soreness didn't just flare up, apparently. Karstens had also been scratched the first time he'd been penciled in to throw batting practice, on Sunday.
On that day, he was rerouted to a bullpen side session, after which manager Clint Hurdle had said, "It was nothing health-related. It's the way we set it up."
Hurdle echoed others' restrained reactions to Karstens' situation.
"There aren't any alarms going off," the manager said. "It's good that [Karstens] was proactive with it. Rather than push the issue by having him pitch batting practice, we had him throw on flat ground and we'll re-evaluate him day to day.
"We'll just slow it down. Our focus is to have him ready in April. He reported it in timely fashion, and we'll see how he feels tomorrow."
Karstens still walked off the field and out of the warm midday sun drenched in the perspiration of a good workout, and reported everything having "felt fine" during his soft toss.
Karstens' timeout, even briefly, dials up the already intense competition for spots in the starting rotation. In the short term, his scheduled stint in Friday's intrasquad game will now go to someone else.
"Ray [Searage, pitching coach] needs to redo his master sheet, and this will provide an opportunity for somebody else," Hurdle said.
Stolmy Pimentel, also scheduled to throw BP on Tuesday, was another scratch.
But with the additional exceptions of Francisco Liriano (broken non-throwing arm) and Charlie Morton (rehabbing from Tommy John surgery), every other pitcher in camp is slated to throw Tuesday or Wednesday.
So all the rotation candidates will parade to Pirate City's mounds while Karstens lays low: Jeff Locke, Kyle McPherson, Justin Wilson, Phil Irwin, even Gerrit Cole and non-roster lefties Kris Johnson and Jonathan Sanchez.