Hefner, who had been in the rotation the entire season, has been ineffective since the All-Star break, going 0-2 with a 9.13 ERA in five starts.
So rather than go to a six-man rotation -- the original plan articulated by the organization with left-hander Jonathon Niese returning from the disabled list -- the Mets decided to demote Hefner.
However, Hefner has been troubled by a nagging elbow injury, the right-hander said. So Hefner will be sent to team doctors in New York to be examined Monday. If Hefner is deemed injured, the demotion will be reversed and Hefner instead will be placed on the major league disabled list, general manager Sandy Alderson said.
Hefner suspects the injury originally occurred when he was struck with a line drive in the right elbow off the bat of Carlos Beltran during spring training. The injury progressively got worse during the season. He indicated he is having difficulty maintaining his stamina deeper in games.
Hefner does not believe the injury was overly serious -- perhaps bone chips, he speculated. He believes his ineffectiveness partly resulted from backing off between-starts work in order to maximize his ability to rebound for his next start.
Hefner is 4-8 with a 4.34 ERA in 24 appearances (23 starts) this season.
"It's been fairly manageable the whole year, but it started to creep up with a little more intensity of late," Hefner said. "I think it's endurance more than anything.
"Going back to the [July 29] Miami game and then the Royals and then the other day against the Diamondbacks, the first three or four innings were usually pretty good. And then it went sour very quickly after that. So I think that's when I felt it the most. Once I get to that 70- to 80-pitch range, I'm completely out of gas and don't feel like I'm as strong as I was.
"There's a little bit of pain, soreness, inflammation -- however you want to call it."
Niese is returning from a partial tear of the rotator cuff in his left shoulder and is due to face the Arizona Diamondbacks on Sunday afternoon in his first major league game since June 20.
Alderson said one reason for doing the demotion in advance of Monday's medical exam is the Mets need to clear a roster spot Sunday for Niese's activation from the DL.
But, Alderson said, with a few outings later in the season from a spot starter, a five-man rotation still should allow for Wheeler and Harvey to mostly complete the season in the rotation without getting shut down because of an innings cap. Hefner, assuming healthy, would be the selection to get a handful of spot starts down the stretch, according to Alderson.
"Particularly with Wheeler and Harvey, who are on innings limits, we think we can still get through almost to the very end of the season with some of the off-days we have," Alderson said.
The GM said there is no Triple-A candidate to be promoted to continue the six-man-rotation plan because those young pitchers are due to hit innings limits soon, too. Alderson noted Las Vegas right-hander Jacob deGrom has thrown 128 innings this season so far -- already a jump from the 111 1/3 innings he logged a season ago.
Fellow prospect Rafael Montero has tossed 131 2/3 innings this season, already a jump from last year's 122 innings.