PORTLAND, Ore. — Dirk Nowitzki was the first one to point it out Monday, saying how dangerous the Knicks are offensively when starting center Robin Lopez is out and 7-foot-3 rookie phenom Kristaps Porzingis is moved to the “5” position. Nowitzki said it stretches the floor and makes the Knicks harder to guard.

With Lopez’s offensive numbers plummeting as he makes his return to face his former team on Saturday, Knicks coach Derek Fisher has mulled whether Lopez’s rim protection and interior play is worth making the Knicks a more lumbering, less-potent offensive club.

Porzingis passed another defensive test Thursday in the pivot in Sacramento when he got better as the game went on in battling center DeMarcus Cousins.

When the Knicks staged their fourth-quarter comeback against the Kings, Lopez was on the bench and Porzingis manned the middle. Lopez sat the game’s final 19 minutes.

Fisher said he’s “evaluating’’ the starting unit and a change could come imminently. Lopez admitted he has struggled to find the right balance in the triangle offense.

“Defensively, that’s been pretty consistent,’’ Lopez said of his first 24 games as a Knick. “I’ve always been able to fall back on that. Offensively I’m trying to figure things out. How much I’m expected to score? What am I expected to do when I catch the ball in the post? Now I’m getting more comfortable, starting to realize what the role is.’’

Lopez scored two early mid-range buckets Thursday, then missed his final five shots. Playing just 20 minutes, he was a team-worst minus-9. Lopez is averaging 6.9 points and 5.5 rebounds. In his two seasons with the Trail Blazers, Lopez averaged 10.5 points and 7.8 rebounds.

If Fisher benches Lopez, it would be a blow to team president Phil Jackson, who gave him a four-year, $54 million pact. The way Fisher’s rotations go, Lance Thomas is a strong candidate to start at small forward with Anthony moving to the 4 and Porzingis going to the 5. Thomas has improved his 3-point range, is a good defender and transition player.

Despite his offensive funk, Lopez looked forward to his Rose City return. He still owns an apartment in Portland and he wears his navy blue Seattle Mariners baseball cap religiously in the locker room.

“Portland was very good to me, the fans, it’s a great city,’’ Lopez said. “I’m excited to go back there. I still want to give them an L.’’

Lopez and the rebuilding Blazers never spoke after July. Management broke up the powerful unit once they knew LaMarcus Aldridge wasn’t re-signing. Wesley Matthews also split for Dallas. The Portland purge started last season when the Blazers shipped Nicolas Batum to Charlotte.

“We did have a good thing in Portland, the two years I was there,’’ Lopez said. “We all appreciate what we did there and all happy where we are now.’’

Arron Afflalo, who played 25 forgettable games in Portland after the trade deadline last season, also makes a return. So does MSG Network’s rookie sideline reporter Rebecca Haarlow, who worked the Blazers sidelines for Portland from 2007-2010. Haarlow, a TNT alumnus, coined Porzingis’ nickname — KP6.