Daisuke Matsuzaka is an early arrival to the Boston Red Sox spring training camp. However, he didn't appear on the practice field today, according to ESPNBoston.com's Gordon Edes. The answer as to why might come from several Japanese media outlets.
According to translations, the Kyodo News, and the sports website Sponichi Annex reported that Matsuzka skipped a throwing session late last week due to tightness in his back. Edes wrote that Matsuzaka's non-appearance on Tuesday likely shows that Matsuzaka's condition hasn't improved enough for him to resume pitching.
The Japanese media reports also indicate that the team is aware of the injury. However, Edes wrote that Red Sox general manager Theo Epstein didn't mention an injury while talking to reporters today about Matsuzaka, despite numerous questions.