Keller had his start at Triple-A Indianapolis scheduled for Sunday pushed back to Tuesday, Stephen J. Nesbitt of The Athletic reports.
The change in schedule was not related to an injury, and the Pirates need a starter for Wednesday's game against Colorado. The 23-year-old prospect has been working on developing a slider at Triple-A Indianapolis, but he could be deemed ready to join the big-league club. He's managed only a 3.83 ERA and 1.50 WHIP this season but has shown improved command of late, walking six in his last 20 innings of work.