The skipper said that Karstens threw 20 pitches to three different hitters, using his full assortment, and did well. Karstens has been on the 15-day disabled list since April 18 with shoulder inflammation.
"He reported that he felt fine today after throwing," Hurdle said before Wednesday's game with the Nationals. "He activated, got loose, pitched to three hitters."
Hurdle said it will be important to see how Karstens feels tomorrow, and then on the follow-up session Friday.
"[That] will give us a better read on what the next step is for him," Hurdle said.
Karstens has started three games this season and has an 0-1 record with a 4.50 ERA.