Not just a Premier League loss, a fifth in their last six games. But the manner of the setback.
Saturday's 3-2 defeat to Everton was pretty much the story of Newcastle's Premier League season wrapped together in 90 minutes.
Summer signings struggling, a porous defence giving away shocking goals, naivety and woeful game management, a muddled and misfiring attack.
Everton weren't great but you don't have to be to beat Newcastle at the minute.
Newcastle have already lost as many games at home this season as they did in the entirety of last campaign, and there are still five more St James' Park fixtures to come. Only two teams, one being bottom club Wolves, have conceded more home goals than Newcastle this season.
Sides used to fear Tyneside trips to face Eddie Howe's side. Not anymore. For the first time under Howe, the Magpies have now lost three home games on the bounce. The soundtrack to the full-time whistle was boos, again.
Wednesday night's visit of Manchester United now feels huge, but so did this.
Newcastle didn't play like it was a game that mattered. The first half an hour was awful, not helped by Howe's selection decision that backfired badly. Nick Woltemade looked lost in midfield and if Howe is to continue with the experiment then the German must be doing something pretty remarkable in training.
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To accommodate Woltemade in midfield, Howe shifted Joelinton to the left and the Magpies badly missed the Brazilian's physicality and power in the middle.
Howe realised it wasn't working and shifted things around after half an hour, but the damage was done. Everton had a goal lead and belief. What they also got was a helping hand from the hosts. After Jacob Ramsey cancelled out Jarrad Branthwaite's opener, Nick Pope gifted Beto a second for the visitors.
Newcastle can't rely on Pope and need to prioritise the signing of a top class goalkeeper in the summer. Everton have one and Jordan Pickford made the save of the season deep in stoppage time to deny Sandro Tonali.
Before that, Jacob Murphy thought he'd rescued a point late on, only for Newcastle to again immediately switch off. Anthony Gordon, below-par yet again against his former side, lost possession and Newcastle lost the game, substitute Thierno Barry bundling home at the back post.
Newcastle used to be the Premier League's bullies under Howe. Too often this season they've been a soft touch.
Forget talk of the Champions League next season. Their only hope of that is to win the competition this season, but play like they did against Everton or Brentford and Barcelona will do to Newcastle what the Magpies did to Qarabag.
But that's the thing, the chances are Newcastle will turn up against Barcelona. They're capable of turning it on and reminding everyone what they can be.
But as Howe said on Saturday evening: "The players know how hard it's going to be and there's an element of raising our game. I don't think that's the sign of a top team. We have to be at that level all the time."
What was so frustrating about Saturday's setback is it came after a promising spell that suggested Newcastle might have turned a corner. They won at Villa Park, dismantled Qarabag in the first leg and lost with their heads held high at Manchester City, which has too often not been the case.
But after one step forward, one back. The Brentford defeat was supposed to be a turning point.
“Certainly in relation to the Premier League, our form for a while has not been strong enough," said "hugely frustrated" Howe.
"It's not been good enough, we know that, we take responsibility for that. It's been really frustrating for us, and that is the perils of Europe, I'm afraid.
"We've tried to be laser-focused on every game and give every attention to every game with the same importance.
“But with the deluge of games, your focus can be swayed and yes, our Premier League return in recent weeks has been nowhere near the level that it has been historically and our home form, which has been so reliable for us, has tailed off.
“We've got to try to grab that back as quickly as possible. We love playing here, we want this to be our place where we pick up the most points, where the players feel free to play, and through our own performances, that hasn't been the case."
Howe added: "We think we're a good team and we think we've got every opportunity to grab games and go on a really good run.
"The team is in a much better place in terms of a general performance than it was at the start of the season but we're not putting it together for 90 minutes, even in games. That's a huge frustration."