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3/18/2014 6:18 P.M. ET

Niese no longer a lock for Opening Day

PORT ST. LUCIE, Fla. -- For Jon Niese, this spring has entailed two flights to New York, two MRIs and two rather sizable sighs of relief.

"I feel like that piece of paper that you stick up on a dartboard and the guy is throwing the knife at it," Niese said on Tuesday. "Luckily for me, the guy throwing the knife isn't a very good shot."

Niese officially returned to camp on Tuesday after his latest MRI revealed no structural damage in his sore left elbow. He expects to resume throwing off flat ground on Wednesday, though he is no longer a lock to start on Opening Day.

Instead the Mets could push him back as far as April 6, when they will need a fifth starter for the first time. That would allow them to open the season with Niese on the disabled list, clearing space for an extra bench bat or reliever.

Regardless of where Niese slots in the rotation, the important thing for the Mets is that he is healthy. Niese, who missed nearly two months last summer with a partially torn rotator cuff in his left shoulder, also sat out earlier this spring with discomfort in his shoulder. There is a chance that his recovery from that ailment placed extra stress on his elbow.

"It's probably from trying to look at the radar gun -- overexertion, just trying to get that extra mile an hour," said Niese, whose velocity was noticeably down after returning from New York the first time. "All of a sudden, mechanically, you go the wrong way one time and it pinches and it's irritated. One thing I'm going to try to stop looking at this spring is the radar gun and just go by feel, because I know if I feel good, the velocity is going to be there."

If Niese does not pitch on Opening Day, either Dillon Gee or Bartolo Colon will take his place.

Harvey tuning out spate of Tommy John candidates

PORT ST. LUCIE, Fla. -- One after another, stories of multiple Tommy John surgeries have littered baseball's recent landscape. A's starter Jarrod Parker is the latest example, headed for a second surgery on his right elbow. Atlanta's Kris Medlen is also scheduled for his second operation, as is his teammate Brandon Beachy. San Diego's Cory Luebke is on the list as well.

Recovering from his first -- and, he hopes, only -- Tommy John surgery, Matt Harvey is doing his best to tune it out.

"I want to be on the field more than anybody, but I also know there is a process," Harvey said. "It is slow. You have to take things very carefully, and everybody's different. My mind-set is always going to be, 'Come back early, and come back and win.'"

With that in mind, Harvey has busied himself with his throwing program, making 20 tosses at 60 feet four times a week. He hopes to increase his throwing distance to 75 feet by the end of this week, keeping him on track to climb atop a mound around June. The ultimate goal remains Opening Day 2015, even if he continues holding out hope for a September return.

"Everybody's always going to be so cautious about the recovery, but as competitors, we're always going to want to get back on the field," he said. "If the doctors at that point say it's fine, my mind's always going to be on the baseball field."

Before any of that can happen, however, the Mets must determine where he will be when camp breaks next week. Though players typically rehab at their teams' Spring Training facilities, Harvey lives in New York year-round and has expressed a desire to rehab there until he is close to returning to the big leagues. MLB's Collective Bargaining Agreement is on his side, stipulating that the Mets cannot force him to train in Florida for more than 20 days without his written consent.

Ultimately, Harvey and the Mets could come to some sort of compromise, perhaps allowing him to rehab in New York when the team is at home, and in Florida when the club is on the road. Those are discussions the two sides will have over the next week.

"Obviously, I want to do what's best for the organization and best for the rehab, and I think at that point it's going to be a discussion of where the best place is at certain times to do things," Harvey said, noting that he discussed his rights with the MLB Players Association earlier this month. "I don't want to be in New York for a story. I don't want to be around [the media] all the time. I want to be there to support the team, be there to learn. And the story is the 25 guys that suit up every day to play."

Worth noting

Wilmer Flores made a highlight-reel play at shortstop in the ninth inning of Tuesday's game, and manager Terry Collins said afterward that -- assuming he does not make the big league club -- Flores will see significant time at the position during the regular season at Triple-A Las Vegas. Collins indicated that Flores will log significant innings at short and second base, but not necessarily third.

• Right-hander Jeremy Hefner continues advancing in his rehab from Tommy John surgery. Hefner, who has been throwing off flat ground since February, on Tuesday increased the distance of his throws to 75 feet. Once he reaches 150 feet, he will be able to throw off a mound for the first time, in his quest to return by September.

• ESPN New York reported that the D-backs have dispatched "an army of scouts" to Port St. Lucie, where on Tuesday they watched starting pitcher Logan Verrett and catcher Kevin Plawecki play in a Minor League game. The D-backs have a surplus of shortstops available for trade, including Didi Gregorius and Chris Owings.

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.