OCEANSIDE, Calif. — The same day Robert Saleh ended his tenure as a New York coach, Jordi Fernandez started his.

Fernandez made his NBA head coaching debut in the Nets’ 115-106 preseason opening loss to the Clippers before a sellout crowd at Frontwave Arena that included team owner Joe Tsai and wife Clara Wu Tsai.

It was just a preseason opener, and every exhibition season comes with the public service reminder that these games mean little.

Even less so with both rosters missing bodies, the Nets without starting center Day’Ron Sharpe and four reserves, and the Clippers sans star Kawhi Leonard.

Cam Thomas had 12 points, and Keon Johnson led the Nets with 14 off the bench.

But seeing the talent-starved Nets fall to their former star James Harden — who couldn’t capitalize on a Big 3 era with Kevin Durant and Kyrie Irving, but had a team-high 14 points all in the first half — was sobering, if not galling.

The Nets have embarked on a rebuild, and are entrusting Fernandez to steer them through it.

“Obviously this is my first game as Nets head coach. And it’s always special. Obviously I’ll always remember that it was here,” Fernandez said. “Like I told our guys, gamedays are awesome. It’s an opportunity to compete and we get to do it together. So it’s a good day to start this. And we’ve got to make sure we bring it every single day, every single night.

“You’ll never forget this game. It’s unique. … Now I get to be a head coach for the first time with a team that gave me the opportunity and believed in me. That is very special … so again, a day that I’ll remember for the rest of my life.”

The game was in some ways a microcosm of the Nets.

Having only been a head coach for Team Canada, Fernandez’s style had been a mystery.

But there was at least a foggy image of how he would like to play when healthy.

The Nets want to run, and had 27 fast break points. They will need to thrive in transition, because their halfcourt offense is a work in progress.

Fernandez was clear what he wanted.

“Our ball pressure defensively, our physicality,” said Fernandez. “And then offensively, how many times we can get to our spots and run and play fast; and not just move the ball but also move bodies. How hard we play is going to dictate how good we’ll be.

“So far in training camp, these guys have been unbelievable with their purpose, with their work ethic, with their togetherness. And that’s what I want to see. I know it’s not going to be perfect: I’m not looking for perfection. I’m just looking for that purpose and to get 1% better. So, if we can come here and get 1% better tonight, I’ll be very happy. And then we’ll move on to the next game.”

The start was ugly.

The Nets coughed up the first 12 points of the game.

Harden’s 3 made it 9-0, and Terance Mann free throw left them in a 12-0 hole just 2:49 into the game.

The Nets didn’t get on the board until Ben Simmons found Dorian Finney-Smith for a thunderous dunk with 8:56 left in the first.

But the Nets still trailed by as much as 18, at 23-5 on a Norman Powell jumper and 26-8 moments later after a Mann pull-up.

But with Simmons forced to play out of position at center thanks to the Nets being without Claxton and backup Sharpe, they trailed 35-18 after the first quarter.

But once Simmons got to play his natural point guard position in the second, the Netsfound some life.

Down 40-29 following Amir Coffey’s hook shot, the Nets reeled off a 16-1 run to get back into the game.

Fernandez has been preaching competitiveness, and veteran Dorian Finney-Smith showed some by diving on the floor for a loose ball, and drilling a 3 seconds later to pull the Nets within 42-40. Cam Johnson capped the run for a 45-42 edge.

But once both teams went to their benches, the Nets coughed up a 21-4 run that spanned the third and fourth.

Up 74-67 after a Jalen Wilson and-one, they couldn’t stem the Clippers’ tide and found themselves back down by double digits.

Still, Fernandez — who spent two years as the Cavaliers’ G League coach in 2014-16 while Ty Lue was a Cavs assistant — has impressed his former co-worker.

“Jordi did a great job. His [Canada] guys were playing hard. They understand what he was running, what exactly he wanted to do offensively and defensively,” Lue said. “He demanded that of all those guys, you see him on the floor, he’s directing guys, he’s getting on guys. So he did a really good job.”