PORT ST. LUCIE, Fla. - No longer confronted with the need to conceal the extent of his injuries, Mets lefthander Jonathon Niese has been an open book this spring training.

Time has brought forth full disclosure.

He already has spoken freely about the sharp pain that cloaked his shoulder last season. And Sunday, after his Grapefruit League debut in the Mets' 6-3 loss to the Red Sox, he revealed why he battled with consistency in his delivery.

"Last year, I was just trying to find an [arm] angle where it didn't hurt," Niese said. "So this year, I'm 100 percent healthy so I'm able to actually work on my mechanics and have proper mechanics."

Manager Terry Collins said the difference has been clear, and for the first time in an exhibition game, Niese seized the opportunity to show it.

He retired the first eight batters he faced before hitting a wall. With two outs in the third, the Red Sox tagged Niese for three straight hits before Daniel Nava worked a walk on Niese's 52nd and final pitch.

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In the regular season, Niese might have fretted about Collins pulling him from the game. But early in spring training, he understood that he had accomplished all he could.

"The arm felt great," Niese said. "Kind of ran out of gas at the end there and lost command. But it was good to get to 50 pitches."

Niese said work remains to be done with his cutter and curveball, but he has passed every important checkpoint. He not only is pitching without pain but insists that he's "cutting loose with everything," more than he could say last year, when he went 9-11 with a 3.40 ERA.

For Niese, who has a spotty injury history, health remains the most important consideration.

"His arm angle is completely different than it has been the last year and a half," Collins said. "You're seeing the ball come out of his hand better."

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That's no small development for the Mets and Niese, the longest-tenured member of the rotation and its lone lefthander.

"He's going to be a big piece," Collins said. "It's nice to have that lefty in the middle of that rotation."

In the context of the Mets' youth movement, Niese, 28, is almost middle-aged. Consider where some of the other members of the 2015 starting rotation stood in September 2008, when he first broke in:

Matt Harvey had just come off his first summer in the Cape Cod League.

Jacob deGrom had been the starting third baseman at Stetson University.

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Zack Wheeler had just given up high school basketball to spend his senior year focused on baseball.

Meanwhile, in Milwaukee, a then-21-year-old Niese was making his big-league debut for the Mets.

Beginning in 2010, he has been a regular presence, making at least 24 starts in each of the last five seasons. In that span, Niese has lived through the Mets' leanest times. But now, if he can stay healthy, he can play a major role in the turnaround.

"I'm excited," he said. "Especially looking at all the talent we have. I came into camp healthy, I feel strong. The sky's the limit for us this year as far as the pitching staff goes. I know our hitting has gotten better. I'm really excited about it."