Georgetown's Jabril Trawick defends as UCLA’s Norman Powell looks for an opening in the Legends Classic. (Frank Franklin II/Associated Press)

For Georgetown fans, the most anticipated story line of Monday’s clash with 11th-ranked UCLA was the return of sophomore forward Otto Porter Jr., sidelined all but six minutes of the season to date after suffering a mild concussion.

For much of the rest of the nation, it was the long-awaited debut of UCLA’s prized recruit Shabazz Muhammad, finally cleared to compete by NCAA officials who held him out of the season’s first three games for violating amateurism rules.

Porter proved himself the more valuable player and versatile man, delivering a double-double to spark the 78-70 upset that earned Georgetown a date with No. 1 Indiana in Tuesday night’s championship game of the Legends Classic at Brooklyn’s Barclays Center.

Junior guard Markel Starks led the Hoyas with 23 points on 9-for-14 shooting. But Porter did it all for his team. Whether handling the ball or not, the 6-foot-8 Porter made a difference nearly every minute on the court, grabbing rebounds and steals, dishing out assists and blocking shots like a human flyswatter to finish with 18 points, 11 rebounds, five assists, five blocks and three steals.

“And then there’s a whole lot of stuff he does that doesn’t show up on paper,” noted a delighted Coach John Thompson III, following the big-time victory for a decidedly young squad. “We’re a better team with him on the court, so it’s good to have him out there.”

UCLA’s Muhammad, honored as the nation’s top high school player last year, scored 15 points and had one rebound in his 25-minute college debut.

Georgetown (3-0) broke open the game with a 12-0 run to start the second half and never looked back despite losing center Mikael Hopkins to foul trouble with more than 10 minutes to play. The 6-5 Jabril Trawick stepped in and made some huge plays to keep the momentum going.

UCLA (3-1) was Georgetown’s first real test this season and presented a formidable defensive challenge, boasting height, depth and a staggering amount of talent.

The Bruins start a pair of 6-10 twins, North Carolina transfers David and Travis Wear, and a 6-9 guard.

And their freshman class alone includes three McDonald’s all-Americans, as well as the hot-shooting Jordan Adams (22 points), who has now scored more than 20 points in each of his first four games — a UCLA record.

But Georgetown brandished more intensity at the outset and hit its first four shots to bolt to a 10-2 lead.

Taking on UCLA was a huge leap for the young Hoyas, who lost their three leading scorers from last season, don’t have a senior among them and eked out single-digit victories over two mid-major teams to start their 2012-13 campaign.

But they made tremendous strides with their zone defense, holding the high-scoring Bruins to 29 first-half points.

UCLA fans applauded when Muhammad entered the game after 5 minutes 48 seconds had elapsed, but Porter blocked the freshman’s first college shot.

With Hopkins, Nate Lubick (11 points) and Greg Whittington (13 points) tied up defending the Wears, Starks’s shooting kept Georgetown close. And Porter grew more assertive as the game unfolded.

“I just let the game come to me and trust the offense,” Porter said.

Georgetown took a 31-29 halftime lead against a Bruins team that had tallied 63 points in its first half en route to a 100-70 drubbing of James Madison in its previous outing, coaxing a broad smile from former Hoya Patrick Ewing, who looked on from a courtside seat.

The message at halftime was simple: Rebound and get defensive stops. And the second half couldn’t have started better for Georgetown, with Porter and Whittington scoring eight unanswered points.

UCLA Coach Ben Howland called a timeout, but Georgetown stripped the ball on the inbounds play, and Starks raced for the layup. A dunk by Hopkins capped a 12-0 run that put Georgetown up, 43-29.

The Bruins pared it to four at one point. But Porter hit a three-pointer to restore double-digit lead. And with an 11-point lead at the three-minute mark, Georgetown slowed the pace to a crawl.

Tip-off for Tuesday’s championship game is 10 p.m.