By TIM ROHANFEB. 26, 2014

PORT ST. LUCIE, Fla. — Spring training is where optimism often reigns, while the cold reality of the regular season is still off in the distance somewhere.

But that was hardly the case Wednesday, when the casual atmosphere of late February was punctured by the announcement that Jon Niese, the senior member of the Mets’ starting rotation and the team’s likely opening day starter, had returned to New York to have a magnetic resonance imaging exam on his left arm.

An M.R.I. on any pitcher’s throwing arm is worrisome, but in the case of Niese it is doubly so because he missed about seven weeks last season with a partly torn rotator cuff. At the time, the Mets concluded that surgery could be avoided and instead had Niese, a left-hander, rehabilitate the area around his shoulder. He returned to the rotation Aug. 11 and made 10 consecutive starts to finish out the season, including a three-hit shutout of Philadelphia on Aug. 27.

But after Niese, 27, reported to spring training this month, he felt pain in the triceps area of his left arm. That pain apparently subsided, Manager Terry Collins said Wednesday, but the discomfort returned when he threw to Mets hitters during a recent batting practice session.

“At the end of it he said, ‘Geez, my arm’s just dead,’ ” Collins said.

The M.R.I. will take place Thursday morning, and Collins characterized the procedure as precautionary. Still, the Mets have every reason to be concerned, particularly since they have been battered by serious injuries to their starting pitchers in recent seasons.

Matt Harvey, their 24-year-old ace, will probably miss all of the 2014 season as he recuperates from Tommy John surgery. Jeremy Hefner, a more modest member of last year’s starting staff, had Tommy John surgery last August and is no longer with the team.

Jenrry Mejia, a solid prospect, had the surgery in 2011, stalling his career. In 2012, Mike Pelfrey had Tommy John surgery, and it cost him nearly the entire season. And Johan Santana, the team’s ace before Harvey, missed the 2011 and the 2013 seasons because of shoulder operations.

The loss of Santana was extremely costly to the Mets because the injuries came while he was under contract for six years and $137.5 million. Niese signed a five-year, $25.5 million contract extension in April 2012, one of the few significant financial commitments Sandy Alderson has made since he took over as the Mets’ general manager for the 2011 season. Niese is scheduled to make $5 million in 2014.

Last season, Dan Warthen, the team’s pitching coach, suggested Niese’s initial injury might have been related to two April starts he made in almost unbearably cold conditions. Pitching in Minnesota on April 12, when the game-time temperature was 34 degrees, and in Denver six days later, when the game-time temperature was 28, Niese ended up altering his delivery, Warthen said, and probably aggravated his shoulder.

Niese, a seventh-round pick in the 2005 draft, made his major league debut three years later and has a career mark of 43-40 with the Mets, with a 3.99 earned run average.

PLEASED WITH TEJADA’S EFFORT At least for now, the Mets keep saying, Ruben Tejada is their starting shortstop. And Sandy Alderson said it again Wednesday.

After publicly questioning Tejada’s work ethic last year, Alderson said he was pleased with the effort the 24-year-old Tejada had shown in his off-season workouts and with what he had done so far in spring training.

To Alderson, it does not matter that Tejada, who was overweight a year ago, does not have a markedly different physique this time around. What is important, Alderson said, is that Tejada looks energized and has put in extra work. Now, Alderson said, he wants to see how that work translates into exhibition games, which, for the Mets, begin Friday.

Stephen Drew, a free-agent shortstop, is still available, and until he signs elsewhere, the Mets will remain a plausible destination. The Mets have characterized that outcome as “unlikely,” however, considering Drew’s contract demands. And Alderson indicated that neither he nor Scott Boras, Drew’s agent, had changed stances.

“There hasn’t been any communication,” Alderson said.

That could change, depending on how the Mets view Tejada in the days and weeks ahead.

“I’ve always felt, from his standpoint, it was more about his mental and emotional approach to the game than his physical capabilities,” Alderson said of Tejada. “Taking all those things into account, I think we’re pleased with where he is.”

Alderson would be more than satisfied if Tejada returned to the way he played in 2011 or 2012, when he batted .284 and then .289 and fielded his position capably.

“We don’t need him to be Cal Ripken Jr.,” Alderson said, adding, “If he can get on base at a reasonable clip, get back to the line drives he’s known for and play consistent defense, that’s what we’re looking for. And he’s capable of that.”