New York Mets pitcher Steven Matz massages his arm in...

New York Mets pitcher Steven Matz massages his arm in the dugout after pitching during the first inning of a rehab outing in a Florida State League baseball game against the Palm Beach Cardinals in Jupiter, Fla., Thursday, Aug. 20, 2015. Credit: Phelan M. Ebenhack

A lingering back injury has jeopardized Steven Matz's chances of making the Mets' postseason roster.

Matz, the former Ward Melville star, was scratched from his scheduled start for the second straight day. He was slated to pitch on Thursday. Now, it's unclear when he'll take the mound.

He was examined Wednesday night and manager Terry Collins believes he has a muscle spasm. Matz is getting medication, and according to Collins, the lefthander is feeling better and could be slotted in this weekend.

Matz could pitch in relief with starters Noah Syndergaard, Matt Harvey and Jacob deGrom set for abbreviated outings.

The Mets also could send Matz to the instructional league in Florida to face batters if he's not ready to pitch by the weekend or if inclement weather forces a postponement. He also could face hitters in a simulated game on Wednesday.

"I'm not very concerned," said Matz, who compared the stiffness to a crick in the right side of his upper back. "It's going to go away. It's just a matter of when."

But the Mets can't afford that kind of uncertainty. If Matz is added to the roster for the NLDS but must be taken off because of injury, he would be ineligible for the NLCS.

"The one thing you don't want to do is activate him for the first series, and then have a back flare-up," Collins said.

Matz (4-0, 2.27 ERA) first complained of the issue on Tuesday. The Mets believed he had just slept awkwardly, causing the stiffness. They pushed his scheduled start back from Wednesday night to Thursday.

But his condition did not improve on Wednesday, prompting the Mets to push him back once more.

"It's still lingering a little bit so we're just going to take it day by day," Matz said.

Sean Gilmartin will start on Thursday in place of Matz, whose lengthy injury history includes nearly two months on the shelf this season with a torn lat muscle.

With weather forecasts looking ominous for the next five days, it's difficult to determine when Matz could pitch.

Thursday's scheduled night game already has been moved to 12:05 p.m. in an effort to sidestep bad weather expected to hit Philadelphia.

Matz, 24, has not pitched in a game since Sept. 24.