Morris, who did not have the opportunity to make his Major League debut in his one day with the club, is 1-2 with five saves and a 2.28 ERA in 23 relief appearances with Indianapolis.
Karstens (0-1, 4.50) took the mound on Monday for his first start since April 17. He was placed on the DL on April 18 and had not pitched since with shoulder and hip issues.
Karstens threw a bullpen session on Thursday, which he called "the best one I've had since Spring Training. Everything clicked."
Karstens' return bumps the rest of the rotation back a day.