The 25-year-old McClanahan (11-5, 2.20 ERA) has made 24 starts and emerged as one of the top pitchers in the major leagues this season. He will undergo further testing on Wednesday.
"Going into the game, something just didn't feel right," McClanahan said. "Just couldn't get loose. Can't put a finger on it."
McClanahan will undergo additional tests but is optimistic he won't deal with a lengthy absence.
"Didn't want to take the risk of jeopardizing the big picture for one or two starts," McClanahan said. "We're actually pretty optimistic about it. Don't think it's anything major."
The Associated Press contributed to this report.