Tohoku Rakuten Golden Eagles closer Yuki Matsui is expected to be out for about four weeks after he was diagnosed with muscle damage in his left shoulder.
The 21-year-old lefty with 29 saves, four shy of his career high, was found to have muscle damage in the back of his shoulder and was deactivated on Thursday.
Matsui, who was not used in a save situation the night before, went to the Eagles training staff before Thursday afternoon's series finale against the Fukuoka SoftBank Hawks.
"I cannot figure out when it began hurting," trainer Daisuke Makita said. "We'll evaluate his condition daily and decide when he can start (pitching again)."
When asked the night before why he employed setup man Hiroyuki Fukuyama to save Wednesday's game, Eagles skipper Masataka Nashida said he wanted to give Matsui some rest to conserve his strength for the final months of the pennant race.
"I think this is due to fatigue," Nashida said after Matsui's situation was clarified.
Matsui's 29 saves are second most in Japan behind Hawks closer Dennis Sarfate's 31.