"He threw the ball well, working on some mechanics," Gibson said. "He threw the ball much better than he did the first time."
Gibson added that Cahill is "way behind [Brandon] McCarthy," who is set to begin a rehab assignment with Triple-A Reno on Sunday.
Cahill has been on the disabled list since July 1 with a right hip contusion and was slowed by shoulder soreness two weeks ago, although the shoulder issue has subsided.
Last week, D-backs general manager Kevin Towers said it looked like Cahill might not be able to return until early August.
This has been Cahill's toughest season in his five-year big league career. Before the injury, he was 3-10 with a 4.66 ERA in 17 starts.
Andrew Owens is an associate reporter for MLB.com. Follow him on Twitter @OwensAndrew. This story was not subject to the approval of Major League Baseball or its clubs.