Crystal Palace progressed to the quarter-finals of the FA Cup at a sun-kissed, slightly distracted Selhust Park. A 3-1 win against opponents who played with 10 men for 85 minutes was comfortable enough in the end. Sadly this south London derby will be remembered instead for a single outstanding act of violence.

It took place inside the stadium this time, and in front of the watching millions, as Millwall goalkeeper Liam Roberts launched a scrambled, wildly dangerous flying challenge into the head of Jean-Philippe Mateta with just five minutes gone. In the process Miller killed his own continued participation in the game, not to mention a large portion of the BBC live broadcast, and could conceivably have done the same to Mateta.

At half-time the Palace chair Steve Parish called it the worst challenge he has ever seen and seemed to be recommending a prison sentence. It wasn’t intentional, of course. But it was seriously reckless. Given the force, the point of contact and the level of danger, it was arguably as bad as these things are likely to get.

Selhurst Park had been bathed in clean crisp winter sunlight at kick-off, crackling with clanky echoey lunchtime excitement. Millwall were allocated 3,781 tickets for this six-mile trip down the south London train tracks, and the away end was duly packed out, and ringed, sternly, by hi-vis stewards and a gaggle of widely-jeered Metropolitan police.

The opening moments saw Millwall dally vaguely down the Palace left for a bit. And then with five minutes gone Roberts ran toward a bouncing forward pass, left his box and kicked Mateta in the head. It wasn’t really a kick, more an assault, martial arts-style, launched full force into Mateta’s face, an act that looked in the moment, not just like a red, but like two reds, maybe three.

Michael Oliver took the easy route, stopping the game and waiting for the video assistant referee. He was duly sent to his monitor, took one look, and flourished a weary, what-was-that kind of red card.

It was a horrendous challenge, and utterly brainless given the pre-match talk from manager and players about keeping their heads, being aware of VAR, which is not in use in the Championship. “You have to keep in mind there are certain things you might not be able to do,” Japhet Tanganga had said in the buildup. Hmm. About that.

Mateta left the pitch on a stretcher. Eddie Nketiah came on. Lukas Jensen replaced Roberts. The game restarted with an empty feeling around the ground.

Oliver Glasner had picked a near-full strength Palace team, with no Adam Wharton, but a high-class attacking unit in Eze-Mateta-Sarr. Josh Coburn, on loan from Middlesbrough, started up front for Millwall with a box-fresh attacking setup behind him. The Dutch-Algerian winger Camiel Neghli made his debut here. Luke Cundle signed for £1m on transfer deadline day.

So much for planning. A man down, Millwall sank back into a low block. And the game was headed one way from that point as Palace had all of the ball and all of the grass, generating a series of close-range scuffles, desperate clearances, last-line wrestling matches. They were sharp down the right where Ismaila Sarr and Daniel Muñoz stayed wide to stretch depleted opponents. With half an hour gone, Ben Chilwell galloped inside Wes Harding and found himself right in on goal, but saw his shot blocked at close range by Jensen.

And Palace finally scored on 33 minutes. It was an own goal scored by Tanganga, who deflected Will Hughes’ whipped left-wing cross powerfully into his own net.

It was 2-0 on 40 minutes, the consequence of another lengthy, mob-handed attack against a defence that just seemed to be sinking backwards. This time Billy Mitchell smashed a clearance against Tanganga, the ball bobbling back to Muñoz, who nipped in and tapped it into the net. The VAR overruled an offside flag. That made it a goal and an assist for Tanganga, who has been excellent all season, but won’t remember this half fondly.

Against not just the head but every single first-half metric Millwall pulled one back 11 minutes into stoppage time. Wes Harding held the ball up after a neat run and cross from Femi Azeez and clipped it into the corner. The half-time stats read: 79% Palace possession, 11 shots to two, 249 passes to 41. And against all expectations, a 2-1 scoreline.

The second half was slow-burn by comparison. Millwall came forward a little more. They had chances, or rather half-chances, with Coburn a lively presence. Palace seemed happy to hold their lead, but made it 3-1 on 81 minutes, Nketiah nipping in to finish, then doing a no-celebration pose in front of the away fans. Nketiah is from the Honor Oak estate, a couple of miles from the Den. It felt, at the very least, like a neighbourly overture, in violent contrast to the opening act at the other end.