According to the Associated Press (h/t ESPN.com), an "overzealous admirer" yanked on Matsuzaka's throwing arm, causing inflammation.
Matsuzaka is a member of the Nippon Professional Baseball league's Chunichi Dragons after playing for the Boston Red Sox and New York Mets in the majors from 2007 through 2014.
Dice-K arrived in the United States to plenty of fanfare in 2007 as Boston's prized signing. After going 15-12 with a 4.40 ERA as a rookie, he enjoyed his best season in 2008, as he finished fourth in the American League Cy Young Award voting with an 18-3 record and 2.90 ERA.
Matsuzaka was never the same after that, though, as he had an ERA of 4.42 or higher in each of the next five seasons. He also underwent Tommy John surgery in 2011, which marked the start of his arm injury issues.
After posting a 3.89 ERA in 34 appearances (nine starts) with the Mets in 2014, he went back to Japan and signed with the NPB's Fukuoka SoftBank Hawks.
Matsuzaka was a dominant pitcher in Japan for the Seibu Lions prior to making the leap to Major League Baseball, and he was twice named the MVP of the World Baseball Classic for Team Japan.
Over three seasons, Matsuzaka made just one appearance for Fukuoka because of injuries, which led to his signing with Chunichi.
Last season, Matsuzaka was named an All-Star and finished with a 6-4 record and 3.74 ERA en route to being named the NPB Comeback Player of the Year.
Matsuzaka was expected to be a big part of the Dragons' success in 2019, but he has been told to cease throwing until the inflammation subsides.