NEW YORK — The root of Zach Eflin's slide remains a matter of conjecture.

But his slider sure isn't helping.

Eflin got roughed up again Saturday night, continuing the breakdown of what once appeared to be a breakout season. The Phillies righthander lasted only three innings and gave up three big extra-base hits, all on sliders that the New York Mets crushed en route to a 10-5 pounding before an announced crowd of 25,094 at Citi Field.

Make it five consecutive duds for Eflin, who has allowed 24 runs (20 earned) on 36 hits in his last 23 1/3 innings for a 7.71 ERA since the Phillies optioned him to triple A on Aug. 11 in a bit of roster maneuvering to keep an extra player on the bench.

In the midst of a playoff race, can the Phillies really keep sending Eflin to the mound?

"I have nothing but confidence in Eflin," manager Gabe Kapler said after the Phillies slipped to three games off the pace in the National League East pending the outcome of the Atlanta Braves' late game in Arizona. "Eflin's going to go out there and make his next start. And I would not be surprised if we rode him to the end of September."

Eflin gave up three runs in both the second and third innings, dropping the Phillies down a hole that proved insurmountable even though they got to Mets ace Noah Syndergaard for 12 hits, the most he has allowed in a start this season. The Phillies stranded 12 runners, leaving the bases loaded in the third inning, when Nick Williams popped up the first pitch, and in the seventh, when pinch-hitting Jose Bautista swung through a high fastball from lefty reliever Jerry Blevins.

With a victory, the Phillies could have clinched their first series victory since a sweep of the Miami Marlins on Aug. 2-5. Instead, they must defeat Jacob deGrom, the Mets' Cy Young Award hopeful who will take his majors-leading 1.68 ERA to the mound on Sunday.

After a dominant first inning, Eflin inexplicably got away from his fastball in the second. The result: He loaded the bases with one out before Mets rookie catcher Tomas Nido jumped on a first-pitch slider for a three-run double.

Eflin recalled shaking off catcher Wilson Ramos' call for a fastball "maybe one or two times," but he said they generally agreed on the slider-heavy game plan, which continued to their detriment in the third inning. Jeff McNeil lined a leadoff triple on a slider, and three batters later, Todd Frazier smacked a 1-2 slider into the left-field bleachers to give the Mets a 6-0 lead.

"I just got away from my fastball," Eflin said. "I was relying on other pitches I shouldn't have been relying on when my fastball was as good as it was in the first inning. For some reason, I wanted to throw a lot more sliders. Kind of just the way the game was going, I thought that was what was best. Turns out it wasn't. I really need to rely on my fastball more. I think I got away from that in the past couple outings."

Eflin continued to insist that whatever disappointment he felt over getting sent to triple A last month — he didn't miss a start but did lose nearly $20,000 during his nine days in the minors — has had nothing to do with his downturn. But that doesn't make his struggles any less stunning, especially considering how well he pitched through the middle of the season.

In seven starts before the all-star break, Eflin went 6-0 with a 2.32 ERA and held opponents to a .218 average and .581 OPS. His success was a major reason for general manager Matt Klentak to believe he could avoid diving into the market for a starting pitcher before the July 31 trade deadline.

Why mortgage prospects for Cole Hamels or J.A. Happ when you have an upstart 24-year-old who only seemed to be getting better?

Maybe it was a mirage. Maybe Eflin, who has thrown a career-high 114 innings at the major-league level, has simply hit a wall. Regardless, Kapler insists his confidence in Eflin isn't shaken.

"I'd like to continue to focus on the fastball command because I really believe that is what will take Eflin — and also, in some ways, [Nick] Pivetta and [Vince] Velasquez — and create the monsters, the guys who are untouchable," Kapler said. "All three of their stuff is so special and so tremendous."

All of which makes Eflin's last five starts even more confounding.