WHAT IT MEANS: Mike Pelfrey’s vacated rotation spot is a problem for the Mets.

For the second straight start since inheriting that role, Chris Schwinden lasted only four innings while surrendering a pair of homers. This time, Chris Johnson took Schwinden deep in consecutive innings to account for five runs.

In the Mets’ final visit to Houston before it moves to the American League, the Astros swept the three-game series, taking the finale 8-1. The Mets finished their road trip 2-4 and slipped to 13-12 on the season.

Johnson finished with a career-high six RBIs, joining Colorado’s Carlos Gonzalez in reaching that total in a single game against the Mets on the trip. Both of Johnson’s homers drove in former Red Sox infielder Jed Lowrie, who reached base nine of the next 10 times he stepped to the plate after being retired in his first two at-bats of the series.

The big question: Will Schwinden get another start?

The intention was for Schwinden to at least hold the spot until Chris Young is ready in about a month, but that may no longer be tenable as Young works back from May 16, 2011 surgery to repair a torn anterior capsule in his right shoulder. Young threw a 75-pitch simulated game Monday.

The question is: In the interim, who exactly could step?

Mets officials rightly have committed to keeping Matt Harvey and Jeurys Familia at Triple-A for most of the season, which leaves Jeremy Hefner -- as a 40-man roster member -- as the more likely alternative to Garrett Olson among Triple-A Buffalo rotation members. Miguel Batista was intended to be the long reliever/spot starter, and he is around. And D.J. Carrasco did log three innings in one of his minor league rehab appearances, last Thursday.

Of course, any upcoming turnover might not be limited to the rotation. Manny Acosta allowed three runs on six hits in two innings in relief of Schwinden. He is out of options and would need to go on waivers before any attempt to outright him to the minors. But Sandy Alderson was swift a season ago in making changes when players underperformed. Acosta has been charged with 11 earned runs in 3 2/3 innings over his past three starts.

PANAMA! Before giving way to Jordany Valdespin at shortstop for the bottom of the seventh with the Mets trailing by seven runs, Ruben Tejada went 3-for-4. It was his fourth three-plus-hit game on the six-game trip. He hit .464 (13-for-28) during the jaunt to Denver and Houston.

WHAT’S NEXT: The Mets are off Thursday, then begin a three-game homestand Friday against the Arizona Diamondbacks. Dillon Gee (2-2, 4.85 ERA) opposes left-hander Wade Miley (3-0, 1.29) in the series opener.