The Mets said that Niese received a cortisone shot, was told to rest for 48-72 hours and was returning to the team's Spring Training camp in Port St. Lucie, Fla., later Monday.
Niese left the Mets' Grapefruit League game against the Cardinals on Sunday after feeling some discomfort in his left elbow in the second inning. He threw 35 pitches and did not return to the mound for the third inning, then flew to New York on Sunday night to have an MRI as a precautionary measure.
Collins said it's uncertain whether the latest setback will cause Niese to miss his first scheduled start of the regular season -- potentially the opener. Collins said if the Mets feel that Niese needs to remain in Florida for an extended Spring Training when the club breaks camp in less than two weeks, then that will be the plan.
"We're not sure if Jon's going to get the pitch count where we need it to be," Collins said. "We'll see. Probably by the end of the weekend, we'll make that decision."
Collins said that the team has not ruled out Niese possibly starting the season opener.
"Until he gets down here and starts throwing, I'm unable to know what will be done," Collins said. "There's all sorts of options. If he needs an extra outing, then he can stay here."
Following his second setback because of an injury this spring, Niese described the past six weeks as the "Spring Training from hell" on Sunday after he left the game. Collins injected some positive humor into the situation following Monday's game and after hearing the positive news earlier in the day.
"I'm not sure how many MRIs you're allowed to have in a month before we start worrying about things," Collins said. "But his arm's been looked at in every which way and direction, and there's certainly nothing in there."
Steve Dorsey is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.