Williamson, 30, started the ninth in the Cubs' 8-5 win over the St. Louis Cardinals. He faced five batters, and had to leave when he felt some discomfort near his elbow.
"He didn't feel it on any of his fastballs," Cubs athletic trainer Mark O'Neal said. "He threw a breaking pitch at the end and it bothered him a little bit."
The right-hander has had two Tommy John elbow surgeries, in 2001 and 2004, but O'Neal said this problem is not related to the ligament reconstruction. O'Neal compared it to tennis elbow, which is a painful inflammation of the tissue around the elbow. Cubs pitcher Carlos Zambrano had a similar problem last year.
"He has good strength, he's got good motion," O'Neal said of Williamson. "Right now, he's sore, and with what we're doing, obviously the best thing is to put him on the [disabled list]. We need people, we need somebody who can pitch two or three days from now, and I can't guarantee that now."
Cincinnati-based Dr. Tim Kremchek performed both of the surgeries on Williamson's elbow, and O'Neal said the pitcher was expected to fly there to be examined as a precaution.
The Cubs were expected to call up right-hander Carlos Marmol from Double-A West Tenn to take Williamson's spot. Marmol was 3-2 with a 2.33 ERA in 11 games, striking out 67 over 58 innings.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.