Matz threw 32 pitches, faced 11 batters and allowed seven runs on eight hits. Callaway said the 27-year-old was experiencing tightness and a "dead arm."
"Maybe he's hitting a little bit of a wall right now," Callaway said.
Matz is 5-9 with a 4.35 ERA in 21 starts this season. He struggled with elbow problems through most of 2017 before having season-ending surgery in August.
Matz has pitched 107 2/3 innings this season. His career high is 132 1/3 innings in 2016, and Callaway says the workload may be a factor.