The Gunners’ meek defending allowed Forest to claim a famous 4-2 win and Wenger’s men ended the game with tempers frayed and reputations damaged.

Victory gave Forest the chance to remind a national audience of the potential of the former European champions. And Wenger pulled no punches after the game in his assessment of a side showing nine changes from their last Premier League game.

“Overall I congratulate Nottingham Forest for an outstanding performance,” he said. “They were sharp, winning decisive challenges and were dangerous going forwards and overall they deserved to win, it is as simple as that.”

Forest youngster Ben Brereton gave Arsenal a torrid afternoon with his pace and power leaving World Cup-winner Per Mertesacker looking lost on a dreadful afternoon for Wenger’s men.

Brereton scored a penalty and Kieran Dowell also struck from the spot after defender Eric Lichaj had been the unlikely scorer of Forest’s first two goals.

Brereton admitted they thought Arsenal were “beatable” when they saw a teamsheet missing so many established Premier League stars, but Wenger stood by his decision to ring the changes.

“We had a very difficult schedule,” said Wenger, whose side head for Chelsea on Wednesday for the first leg of their League Cup quarter-final. “Many players were on the edge, we play on Wednesday, so it was nearly impossible.

“I understand the selection will be questioned but we had eight or nine experienced internationals on the pitch. With all the respect, we played against Championship team and that would be the wrong excuse.

“I don’t like to talk about individual performances. It is disappointing we lost the game and that’s all I can say.

“They were better than us. We have another big game on Wednesday and must focus on that.”

Wenger endured the agony of defeat from the City Ground directors’ box as he began a three-game touchline ban for comments about officials.

He admitted he “hated” the experience so was not about to launch another rant at referees despite a string of key decisions that went against his side.

They conceded the opener on 20 minutes when Dowell was tripped on the right-hand edge of the area and his left-footed free-kick was bundled in at the near post by Lichaj.

Arsenal took just three minutes to equaliser, however, as Reiss Nelson won a free-kick on the left, Walcott delivered it, Rob Holding headed against the woodwork and Mertesacker stabbed home the rebound.

But Forest restored their lead a minute from the break as Arsenal failed to properly clear a Matty Cash cross and the ball dropped for Lichaj, who beat Ospina with a fantastic lob into the top corner.

The hosts’ third goal arrived on 65 minutes when Rob Holding tripped Cash in the box and Brereton drilled home the penalty for the goal his superb display deserved.

A superb Cup tie got even more dramatic when Forest goalkeeper Jordan Smith threw the Gunners a lifeline with 11 minutes remaining, inexplicably letting a through-ball slip through his grasp for Danny Welbeck to tap into an empty net.

But six minutes later the game was up for Wenger when he saw Armand Traore, a former Arsenal trainee, tripped by Mathieu Debuchy with Dowell taking the ball from Brereton and slotting home Forest’s second penalty of the game.

The ragged Gunners lost their heads in the aftermath as they claimed in vain that Dowell had slipped and struck the ball twice, with Ospina lucky not to be sent off for his protests having been booked moments earlier for contesting the penalty award.

Still there was time for more drama as Forest’s Joe Worrall was dismissed for a late challenge on Chuba Akpom before Walcott sent a lob just off target in a frantic finale.

But Forest held on for a famous win that was earned with five of the club’s former academy players in the starting XI.

Caretaker boss Gary Brazil said: “There’s been lots said about the academy players but the senior players led by example.

“The plan was adhered to well. We filled spaces and made them go round the outside rather than through us. When we had to defend one-on-one, both full-backs were excellent and Jordan didn’t really have a save to make.

“We asked them at half-time not to sit back, to go and get another goal and they did that.

“The players should go and have a good night and relax but, more importantly, the fans should enjoy the occasion and hopefully remember it for a long time.”

Nottingham Forest (4-2-3-1): Smith; Lichaj, Worrall, Mancienne, Traore; Osborn, Vaughan; Cash (McKay 87), Dowell (Mills 90), Clough (Walker 56); Brereton. Sent off: Worrall. Goals: Lichaj 20, 44, Brereton 64 (pen), Dowell 85 (pen).

Arsenal (4-3-3): Ospina; Debuchy (Akpom 87), Mertesacker, Holding, Maitland-Niles; Iwobi, Elneny, Willock (Nketiah 65); Walcott, Welbeck, Nelson. Booked: Debuchy, Ospina, Mertesacker. Goals: Mertesacker 23, Welbeck 79.

Referee: J Moss (West Yorkshire).