It has been a season-long mantra from Rick Pitino.

St. John’s isn’t close to good enough defensively, and it’s going to take time for it to improve in that critical area.

It’s not happening fast enough.

The Johnnies’ defense let them down early and often on Sunday, leading to an ugly 86-80 loss to Boston College at Barclays Center in the NABC Brooklyn Showcase.

St. John’s couldn’t guard the ball. It couldn’t keep Eagles off the free-throw line.

And it dropped a résumé-denting contest to a team picked to finish 12th in the ACC after building a 10-point lead early in the second half.

“We are not going to win until we get committed to defense, and this group is not,” fumed Pitino, who cut off his postgame media session after five minutes to avoid saying something he would regret. “It’s partially our fault. We recruited offensive basketball players. [We’re] trying to teach 23-year-olds about defense and it cost us tonight.

“If you take all the offensive rebounds [we allowed] and the fact that they shot 56 percent, they’re almost scoring every time down the floor. We’re trying to win with offense and you can’t win with offense. So we’re disappointed — very disappointed.”

St. John’s (6-3) was outscored 25-14 over the final 7:39, as Boston College (8-3) shot 56.4 percent from the field, notched 16 second-chance points and attempted 25 free throws after halftime.

The Johnnies led 66-61 with 7:56 left to play, but they were overwhelmed down the stretch — similar to the loss to Dayton in the Charleston Classic.

In five games against top-100 teams, St. John’s is allowing an average of 79.4 points and has a 2-3 record in those contests.

The defense is ranked 127th in the country in efficiency.

Pitino called the halfcourt defense “abominable.”

Asked if St. John’s has the athletes to be a strong defensive team, Pitino made it clear he doesn’t believe it is a talent issue.

Plus, he joked, you can’t make any in-season trades.

“Defense is a matter of will, you got to want to play defense,” the Hall of Fame coach said. “It’s frustrating. But that’s what the first year is, it’s all frustrations. That’s what you get. … It’s very difficult. When you have a 10-point lead, you’re not supposed to lose it. It’s been difficult from the summer to now with defense. It’s a broken record.”

St. John’s didn’t make any players available after the defeat.

Joel Soriano led St. John’s with 21 points and 11 rebounds and Chris Ledlum added 16 and 11, but the backcourt duo of Jordan Dingle and Daniss Jenkins shot a combined 8-for-28 from the field.

St. John’s struggled mightily without Jenkins (10 points, eight assists), who fouled out with 2:11 remaining, outscored by 10 in the 14 minutes he wasn’t on the floor.

“It’s like a ship without a captain when he goes out of the game,” Pitino said.

Five Boston College players scored in double figures, led by Quinten Post’s near triple-double of 14 points, 11 rebounds and nine assists.

The 7-footer created problems for St. John’s from the perimeter, repeatedly finding teammates for backdoor layups.

After a poor first 20 minutes, St. John’s started the second half extremely well, ripping off a 14-0 run to build the game’s biggest lead of 10.

Soriano was instrumental, scoring eight of those points. At one juncture, St. John’s scored eight points in a head-spinning 34 seconds.

The momentum didn’t last.

After a timeout, Boston College answered with a 13-2 spurt to go ahead that coincided with some iffy possessions in which St. John’s settled for long, contested jumpers.

The Eagles kept coming, going ahead by five after freshman Brady Dunlap’s turnover led to a Prince Aligbe dunk with 10:20 left.

It didn’t get better, even though St. John’s did briefly reclaim the lead. It just couldn’t get nearly enough stops.

“We’re going to continue to work at it. We let this one get away,” Pitino said. “We’ve played defense against certain opponents and came away with nice victories. They don’t absorb scouting is the biggest issue. Even underneath out-of-bounds plays. [Boston College] scored three buckets and we played it incorrectly.

“As disappointing a loss as I’ve had, not because of the score, not because of losing. It’s the way we’re losing.”