"Right now, he's going to go back and pitch in that bullpen," manager Terry Collins said, "and hopefully he pitches as well as he did earlier this year."
After Sunday's 2-1 loss to the Padres, the Mets officially activated Niese from the DL and designated righty reliever Buddy Carlyle for assignment.
Matsuzaka proved invaluable out of the 'pen in April, serving as a long reliever, setup man and everything in between. He began to struggle in May, before joining the rotation when Gee went on the disabled list with a strained right lat. Then he remained in the rotation when Niese hit the DL with a sore left shoulder.
Now both starters are healthy, joining a rotation that also includes Zack Wheeler, Bartolo Colon and Jacob deGrom. The Mets know that starting five is unlikely to stay intact all season, and as long as top prospect Noah Syndergaard continues to struggle at Triple-A Las Vegas, Matsuzaka would be the favorite to return to the rotation should anything unexpected occur.
But right now, he is a reliever by default.
"I don't have a crystal ball to tell you what's going to happen in the next five or six days," Collins said. "I really don't know. But as of right now, he's going to go to the bullpen. He did a great job for us there. He pitched very well as a starter. I said, 'You fill a lot of holes for us.'"
Matsuzaka is 2-3 with a 4.24 ERA as a starter this season, and holds a 2.22 ERA as a reliever. Out of the bullpen, he is striking out more than 10 batters per nine innings.
"That's the role he's going to fill," Collins said, "and I think he brings a real useful piece to our pitching staff."
Anthony DiComo is a reporter for MLB.com. Follow him on Twitter @AnthonyDicomo. This story was not subject to the approval of Major League Baseball or its clubs.