"It's not serious," Cincinnati general manager Walt Jocketty said after checking with the Louisville trainer.
The 22-year-old allowed three hits and no runs in 5 2/3 innings, striking out seven and walking three. He threw 96 pitches, 63 of them for strikes, and picked off two runners.
Chapman (4-2) got credit for the 4-2 victory while lowering his ERA to 3.98 in eight Triple-A starts. He has 48 strikeouts and 24 walks in 40 2/3 innings.
In his previous start at Rochester, Chapman allowed eight earned runs and nine hits over 3 1/3 innings. He was pulled during a six-run fourth in which he allowed a three-run homer. Rochester won that game, 13-4.
Chapman posted a 1.29 ERA in his four April starts, but has a 6.86 ERA in four starts in May.