St. John’s will have a hard time making a postseason run without its second-leading scorer. It’ll be impossible without the ability to handle success.

Playing without Mustapha Heron (sore right knee) for the second straight game, the Red Storm followed their vital win over Seton Hall with a near-wire-to-wire letdown loss to Xavier, 84-73, in Thursday’s home finale at Carnesecca Arena.

St. John’s (20-9, 8-8) is now 2-7 in Big East play following a win and has failed to win more than two games in a row since its soft non-conference schedule ended.

“We have to lock in and get better and realize that we’re playing for something more than just Big East play,” senior Marvin Clark II said. “In the [NCAA] Tournament, it’s one game. It’s one game that you’re promised. After that you choose your fate. We gotta lock in and figure it out or we put in all this work for nothing, because that means we go and play one game and we get sent home. We gotta figure out how to string some things together.”

Once again the undersized St. John’s frontline was exploited, with the Musketeers (16-13, 8-8) holding a 16-rebound edge and converting 22 second-chance points. Naji Marshall scored a career-high 31 points.

The Red Storm encounter the same problematic matchup in their final two regular-season games, at DePaul, and Xavier, which has now won eight straight meetings against St. John’s.

“DePaul basically did this to us. This was a replay of what happened to us here,” coach Chris Mullin said. “They imposed their will physically and put us back on our heels, and dictated the game and it stayed that way.”

St. John’s led for just 25 seconds the entire night and suffered its fourth home loss as a favorite in league play, adding to its collection of no-shows in low-profile matchups.

The Red Storm struggled to get in transition or operate against Xavier’s zone defense, and trailed 38-30 at halftime. Justin Simon committed six first-half turnovers and leading-scorer Shamorie Ponds failed to make up for Heron’s absence, while being held without a point until the final minute of the half.

“We need more from everybody,” Mullin said. “[Heron’s] one of our best players. You take one of your best players off it’s gonna put more demand on other guys and a smaller margin of error.”

Behind the standard boost from LJ Figueroa (23 points), and unexpectedly strong showings from Bryan Trimble Jr. and freshman Josh Roberts (making all four shots for a career-high eight points), the Red Storm cut a double-digit deficit to three with 10 minutes remaining.

The unraveling began with a 17-4 Xavier run and peaked with Mullin and Ponds (13 points, 5-of-16 shooting) picking up technical fouls, following a series of crowd-incensing calls and ever-growing free-throw disparity.

Xavier hit 25-of-33 from the line. St. John’s made four-of-five.

“I feel like we did what we had to do to get back in the game,” Clark said. “I feel like everything else is out of our hands.”

How long the season will last still is.