The Gunners were unceremoniously dumped out of the competition they have won more times than any another team, with Lewis Grabban’s 83rd-minute winner no more than the Championship side deserved.
They were the better side throughout the game and thoroughly deserve their place in round four, where they will face holders Leicester in a derby clash.
Arsenal, on the other hand, were alarmingly off the pace and their performance was summed up by the fact that they failed to muster a single shot on target.
Mikel Arteta’s side just never got going and, ahead of a trip to Anfield on Thursday, this was a worrying display.
Left-back Nuno Tavares was hooked after 35 minutes, such was his poor performance, but in truth he was just the worst of a very lacklustre team. By the end of the match, other than Bernd Leno, no one from the Arsenal side could really hold their heads high.
In the first half the Gunners had plenty of possession, around 60 per cent, but they couldn’t create anything. The fact Ben White had their best chances, dragging a shot wide and being pulled down for a penalty shout, summed up the display.
The second half wasn’t much better and, when Arsenal finally created something, Eddie Nketiah could only head harmlessly wide after Bukayo Saka had picked him out.
Arteta was getting more and more animated on the touchline as the game went on, but the performance stayed flat throughout the 90 minutes. The passing was sloppy and, eventually, Forest got the winner they deserved.
The hosts won the ball in midfield and pounced down the right, with Grabban sliding in to score at the back post.
The City Ground erupted and fans celebrated wildly, revelling in their giant-killing. Arsenal, on the other hand, are out of the FA Cup in the third round for only the second time in the last 26 seasons.
Full-back drop-off a clear concern
Nuno was hooked just 35 minutes into this defeat, but Cedric Soares on the other flank was hardly much better.
The Portugal international struggled at the City Ground and had a difficult night. Cedric was sloppy in possession and positionally he was vulnerable, too.
Cedric and Nuno were handed starts as Kieran Tierney and Takehiro Tomiyasu made way, and the drop-off in quality was clear to see.
Tierney and Tomiyasu have been vital to the way Arsenal play this season and that was underlined against Forest.
Arsenal couldn’t build from the back without them and the fluidity to their play was lacking.
Midfield looks light ahead of crucial January
While it was exciting to see Charlie Patino make his first start for Arsenal, the fact he was playing at Forest underlined how short the Gunners are in midfield.
Due to Granit Xhaka being out with Covid-19, Albert Sambi Lokonga is now the only fit, senior central midfielder at the club.
Arteta has called for reinforcements there and this performance demonstrated why that seems vital.
Arsenal have crucial games in January - including two against Liverpool and one at Tottenham - and they don’t look equipped to negotiate that run right now.