Karstens has been on the 15-day disabled list since April 18 with right shoulder inflammation and was scheduled to be activated this week before tweaking his hip flexor during a rehab start on June 7.
"Everything is checked off the list for today," Karstens said on Thursday. "Just looking to come back healthy. I'm tired of being hurt. I'm not paid to be hurt, I'm paid to pitch. And I feel like I'm not doing that right now."
Manager Clint Hurdle said Karstens threw 15 pitches, all fastballs, during Thursday's session.
If Karstens feels well on Friday, he will throw all of his pitches in a 35-pitch bullpen session on Saturday. That would set him up for a Tuesday rehab assignment, from which point he would have 30 days to return to the active roster under Major League Baseball rules.
With the news that Charlie Morton underwent Tommy John surgery on Thursday, Karstens is feeling a greater sense of urgency to return to a starting rotation that has been struggling on the back end.
"I'd be lying if I said there wasn't, but at the same time I have to make sure that I get back to pitching the way I can," Karstens said. "I'm in a good place right now, and hopefully all goes well and I'll be back soon."