"I wanted to keep it quiet. It's just a precautionary thing to make sure it's not worse from last year," Niese told ESPNNewYork.com. "I just want peace of mind."
Niese landed on the disabled list June 21 with a partial tear in his left rotator cuff. He did not return until Aug. 11.
Manager Terry Collins indicated the issue appears to be in the triceps and not the shoulder. But Niese said: "I guess we will see what the MRI says."
Niese, 27, signed a five-year, $25.5 million deal on April 4, 2012, that includes two team options. He went 8-8 with a 3.71 ERA in 24 starts last season but missed nearly two months.
He started on Opening Day last season. The Mets again had penciled him into that role for March 31 against the Washington Nationals.
The plan, barring a disruption because of injury, is to pitch Bartolo Colon in Game 2 and Dillon Gee in Game 3 against the Nationals, followed by Zack Wheeler opening the second series, against the Cincinnati Reds.
Niese's shoulder, tender early in camp, flared up while throwing his latest batting practice session.
"Batting practice started and he felt fine," Collins said. "And then he threw his second 20 pitches the other day. He just said at the end of it, 'Geez, my arm's just dead. Like, it's dead.'
"It's a precaution right now, but any time you're sending someone for an MRI, obviously there's going to be a concern until you get the reading back. He has had in the triceps area a little bit of pain. So we shut him down off the mound and made him throw long toss. That forces you to get your arm a little higher. And he felt good. And then he threw the batting practice the other day and in the second 20 pitches he said his arm just kind of felt like it was a lot of work.
"He just said he's got a little discomfort in there. So we're going to go have it looked at."