Matsui said there is some inflammation, but he couldn't point to a specific instance when the pain started.
"I think it was just something that probably built up starting from about yesterday," he said through an interpreter.
Yankees general manager Brian Cashman said Matsui will meet with Dr. Stuart Hershon on Wednesday to look at the swelling and drain some fluid from the knee.
Matsui underwent arthroscopic surgery on his right knee after the 2007 season, and he said his main concern is that the current soreness isn't as serious.
"It doesn't feel that way, so hopefully not," he said. "Last year was much more painful and that continued on pretty much throughout the season."
Matsui has been a consistent contributor to New York's offense, hitting .321. He's spent most of the season at designated hitter but has also played in left field.
Manager Joe Girardi said he's not sure if the soreness could be due to Matsui playing the outfield against Houston during the team's recent road trip.
Matsui will follow the advice and suggestions he receives from trainers and doctors, but he agreed with Girardi that he should be available to pinch-hit in Wednesday's game if the Yankees need him.
"We thought we'd just give him a day off and see how he is," Girardi said. "It's just a day to try to get him feeling better."
Samantha Newman is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.