Chatwood held the Reds to one run and four hits, struck out four and surrendered one walk in four innings. Chatwood had gone 15 road innings without allowing a run before the Reds' Todd Frazier singled in a run with two outs in the fourth.
The injury occurred while Chatwood was facing Shin-Soo Choo in the third inning, an at-bat that finished in a swinging strikeout.
"It was in the third inning, a fastball to Choo, then after that every time I tried to go glove-side, I felt it," Chatwood said.
The Rockies, who are carrying an extra pitcher and one less position player, needed to save a substitution, so they let Chatwood bat in the top of the fifth. However, they ordered him to bunt, to make sure he did no further damage to his arm.
Chatwood (3-1) has posted a 2.14 ERA in six starts over two separate callups from Triple-A Colorado Springs.
"Boy, that was huge to sort of break the ice [in the fourth], because that young man over there, I don't know what he hurt, but he was throwing the ball great, and he was mixing speeds up," Reds manager Dusty Baker said. "For a young pitcher, he showed great command and control of all his pitches."
Right-hander Rob Scahill replaced Chatwood.
Thomas Harding is a reporter for MLB.com. Read his blog, Hardball in the Rockies, and follow him on Twitter @harding_at_mlb. This story was not subject to the approval of Major League Baseball or its clubs.