Georgetown forward Nate Lubick slams home two of his 13 points against Liberty. Lubick was one of four Hoyas in double figures. (John McDonnell/The Washington Post)

Even with sophomore forward Otto Porter Jr. sidelined with a mild concussion, Georgetown had every possible advantage over Liberty entering Wednesday’s mismatch at Verizon Center — height, speed, skill and depth among them.

But after soundly outplaying the Flames in the first half, the Hoyas showed the peril of their youth in an inattentive second half, surrendering a barrage of three-point shots to let what had been a 23-point lead get slashed to single digits.

In the end, Georgetown prevailed, 68-59, to take a 2-0 record and questionable momentum into the championship rounds of the Legends Classic at Brooklyn’s Barclays Center, where No. 13 UCLA awaits on Monday followed by either top-ranked Indiana or Georgia on Tuesday.

Sophomore forward Greg Whittington led all players with 18 points and nine rebounds. Three others finished in double figures for the Hoyas: Mikael Hopkins (13 points), Nate Lubick (13) and freshman D’Vauntes Smith-Rivera (11).

Still, it fell short of a feel-good victory, exposing cause for concern given the looming potential challenge of facing two nationally ranked teams in two days. Chief among them: a lack of defensive intensity and inconsistent shooting.

Liberty (0-3) was drubbed by Richmond, 84-42, in its season opener and entered Wednesday’s game losing by an average of 28 points per game. But Liberty outscored Georgetown 34-27 in the second half, finishing with 10 three-pointers on 18 attempts overall.

“They got Sunday-down-at-the-park shots,” said Coach John Thompson III, lamenting his team’s lack of attention to detail on defense.

Moreover, Liberty was nearly a half-foot shorter than Georgetown at every position and started only one player taller than 6 feet 5, but the Hoyas barely won the rebounding battle, 29-25.

And Georgetown’s shooting was hot and cold: 62 percent in the first half; 36 percent in the second half.

Said Liberty Coach Dale Layer: “We probably got Georgetown on a night that they weren’t quite as inspired as they will be later in the year. It’s understandable. . . . It took every bit of our best to hang in there and not [have it] be a 30-point blowout.”

Thompson’s analysis was slightly different.

“I don’t want to use this as an excuse or crutch, but we are a young team,” said Thompson, whose squad lacks any seniors and relies heavily on a nucleus of sophomores. “We’re growing; we’re just growing up. Our schedule is such that we have to grow up pretty fast.”

Regarding the 6-8 Porter, a preseason all-Big East first-team selection, Thompson confirmed that he had suffered a mild concussion after being hit on the head during Sunday’s season opener against Duquesne. Porter left that game after just six minutes and didn’t return, and Wednesday night he looked on from the bench, held out as a precaution. Thompson said he thought Porter would be available for Monday’s game, noting that the team’s medical staff would decide.

Sophomore guard Jabril Trawick took Porter’s spot in the starting lineup.

After spotting Liberty four points at the outset, Georgetown reeled off 17 consecutive points, turning the Flames over five times in seven possessions.

Without the size to mix it up in the paint, Liberty played almost entirely on the perimeter. While the Flames managed five three-pointers in the first half, they could do little defensively against the 6-8 Whittington and 6-9 Hopkins.

With a dunk by Lubick followed by a fast break finished by Whittington after a nice outlet pass from Markel Starks, the Hoyas closed the half with a 41-25 lead, having outscored Liberty 24-0 in the paint and tallied 12 fast-break points to Liberty’s none.

But the Hoyas’ intensity waned late in the second half, and Liberty went on an 8-0 run.

“We have to go into every game acting like it’s the last game we’re going to play,” Lubick said. “We’ve kind of struggled to do that these last two games. I don’t think our sense of urgency has been there.”