HOUSTON — Fernando Mendoza’s welcome-to-the-NFL moment was bound to happen.

Almost right on cue, it arrived Thursday night in his first NFL start.

And it was painful.

An 80-yard pick six by Texans linebacker Wade Woodaz in the first quarter of the Raiders’ preseason game against Houston.

Ouch.

What made it sting even more was what immediately preceded it.

Mendoza had just delivered one of his best throws of the game, a 22-yard completion to Jalen Nailor, and fellow rookie Mike Washington Jr. followed with a 33-yard run to the Texans’ 24-yard line.

The Raiders were rolling. Mendoza was settling in. And for a moment, everything looked easy.

Then came the reminder that, for a rookie quarterback in the NFL, it rarely is.

One play after Washington’s big run, Mendoza went right back to him, this time through the air.

Bad decision. Worse throw.

Mendoza fired the ball behind Washington and directly into the arms of Woodaz, who stepped in front of the pass and suddenly had nothing but green grass between himself and the end zone.

Mendoza immediately gave chase, staying with Woodaz as long as he could before launching himself in a desperate diving attempt to prevent the touchdown.

No chance.

Woodaz never broke stride, leaving Mendoza sprawled on the turf as he finished off the 80-yard return and put the Raiders in a 14-0 hole.

Welcome to the NFL.

And, really, welcome to the inevitable.

These are the moments that come with developing a young quarterback, even one as talented and advanced as Mendoza has looked since arriving in Las Vegas as the first overall pick in the draft.

There will be impressive throws and encouraging drives. There will be flashes that make everyone wonder how quickly he can become the Raiders’ starting quarterback.

And then there will be nights like Thursday, when one bad decision and one poorly placed throw become six points going the other way.

Better to experience it in August than October.

And that, in many ways, explains why Kirk Cousins is here.

The Raiders don’t need Mendoza to learn every NFL lesson while simultaneously carrying the burden of winning regular-season games. Cousins gives them a bridge between where Mendoza is today and where they believe he eventually will be.

The veteran can run the offense and absorb the week-to-week pressure of being an NFL starting quarterback while Mendoza develops without the Raiders having to rush the process.

That doesn’t mean Mendoza will spend the season wrapped in bubble wrap. Quite the opposite. Barring some miracle Raiders run under Cousins, Mendoza’s time is coming sooner rather than later this season.

It’s why the Raiders started him Thursday and gave him the entire first half. With Cousins on track to open the season as the starter, Mendoza’s opportunities for meaningful game reps are about to become scarce.

So, these preseason snaps matter.

The Raiders need Mendoza to experience the speed of an NFL defense, work through pressure, make mistakes and, most importantly, learn what happens when he makes them.

Thursday provided all of that.

The interception was the type of mistake Mendoza largely avoided throughout training camp and his preseason debut. But virtually every young quarterback eventually makes one like it. The important part now is what he learns from it.

Mendoza finished 8-of-15 passing for 87 yards while helping guide the Raiders on a field-goal drive. His passer rating was 46.6, and the Raiders trailed 20-3 when his night ended.

Even those numbers require some context.

Rookie wide receiver Malik Benson dropped what would have been a sizable gain and first down on a well-placed third-down throw from Mendoza. Later, Benson had another potential third-down reception poked out of his hands for an incompletion.

Both went on Mendoza’s stat line.

So did the interception.

And unlike the drops, there was no explaining that one away.

It was Mendoza’s mistake, his lesson and another necessary step in the education of the quarterback the Raiders believe can eventually lead them out of the darkness.

The encouraging part for Las Vegas is that he doesn’t have to do it tomorrow.

That’s what Cousins is here for.