If this was not the worst defence of the FA Cup in the modern era, it was in the top one. Arsenal, or at least a team in Arsenal shirts, were run off the park by a Nottingham Forest team so sunk in Championship mid-table that they have just sacked their manager.

This was one of those games where the clichés are appropriate: a raucous FA Cup tie where form and status were made completely irrelevant. What mattered was that this was a hungry, driven, dynamic, young Forest team who shredded Arsenal with every attack. Yes, two of their four goals were penalties, and Arsene Wenger had his own issues with the second, that made it 4-2. But to focus on the referee would be to do an injustice to the gulf between the teams.

Because as good as Forest were, Arsenal were abysmal. Arsene Wenger chose to play what is effectively his Europa League team here, given this was sandwiched between two games against Chelsea, in the Premier League and EFL Cup. Of course, they are coming off the back of a busy Premier League period. But this performance, from a mixture of youngsters and fringe players, was a disgrace. They never got on the ball, they created nothing from open play – their goals were from a free-kick and an error – and their defending was disastrous. The only Arsenal player to emerge with any credit was David Ospina, whose first-half saves stopped Forest from running away with it.

This was not the first time that an FA Cup holder has fallen at the first hurdle. Liverpool were the last team to do it, losing to Bolton Wanderers in 1993. But this felt like the worst-ever attempt, by an Arsenal team utterly devoid of leadership, fight or organisation. The problem, or at least one of them, was that Wenger put the ageing Mathieu Debuchy and Per Mertesacker into the back four, for some baby-sitting duty, if nothing else. But while Ainsley Maitland-Niles and Rob Holding are quick, Debuchy and Mertesacker are not, at least not anymore. Up against this fast young Forest front four – the eldest of whom was 22 – they simply could not keep up.

Eric Lichaj celebrates scoring his side's first goal (Getty)

It is no exaggeration to say that Forest could have scored five goals in the first half. Wenger was exasperated afterwards that his players kept making the same mistakes. The warning was there early on when Ben Brereton raced past Mertesacker and forced the first save of the afternoon from Ospina. But Arsenal continued to play an unsustainably high line and soon after it happened again: Brereton cut in from the left and Ospina had to get down to deny Matty Cash.

Arsenal were rattled and just could not keep up with the pace of the game. They gave away a free-kick out wide, Kieran Dowell whipped it into the near post and there was no-one to stop Eric Lichaj thumping his close-range header past Ospina. To struggle in open play is one thing, but to concede like this from a set play is even worse. It made you wonder what was the point of playing Mertesacker if he cannot prevent goals like this?

As it happened, Mertesacker was the man to score the out-of-the-blue equaliser three minutes later. Reiss Nelson, playing wide on the left, won a free-kick. Theo Walcott curled it in, Rob Holding’s header hit the post and Mertesacker stabbed in the rebound. He ran back to his half with the joyless focus of a man who knew he had much more important and more difficult work to do at the opposite end of the pitch.

Per Mertesacker pulled his side level (Getty)

Mertesacker was right: he did have a lot more defending to do, and it was only going to get worse. Brereton skipped past him yet again, cut onto his left foot and it took a very good save from Ospina, going down quickly to his right, to keep the score level. Straight after Ospina made yet another save to deny the same man.

Forest were flooding forward and the only question was whether the half-time whistle would come before they went back ahead. All they needed was one more chance. Again, Brereton met a cross from Cash, Arsenal weakly tried to clear but it fell to Lichaj on the edge of the box. He chested the ball and volleyed it into the top corner, a goal of ludicrous quality, leaving Ospina no chance.

What was so striking is how there was no real second half response from Arsenal. They looked happy enough just to repeat the dismal first half, as bizarre as that might sound. The game became about whether Brereton could finally score – he nearly put in Tyler Walker’s near-post cross – until Arsenal were generous enough to gift him a penalty. Rob Holding clumsily brought down Cash and Brereton seized his chance from the spot. He sent Ospina the wrong way and Forest were 3-1 up.

Ben Brereton converted from the spot to make it 3-1 (Getty)

The game was as good as lost but Arsenal did scrape an unlikely goal from their only real attack of the second half. Welbeck, who had barely been in the game, found himself in space and shot, Jordan Smith got a hand on it and ball trickled into the net. An ugly goal and as good as it got for Arsenal.

Forest did not let that deter them from their task. Armand Traore surged down the left, past Mertesacker and then past Debuchy. He tried to make a tackle but brought Traore down, and Jonathan Moss awarded his second penalty of the afternoon. Dowell took it, slipped, but the ball looped up and into the net. Arsenal players insisted he touched it twice but the goal was given.