Per Daniel Taylor of the Guardian, he said:
"I just think we were not good enough anywhere—not up front, in the middle or at the back—and we paid for it. We repeated the same mistakes; once, twice, three times.
"[Forest] looked sharper. They had more chances and an outstanding performance. They were sharp, winning decisive challenges, and dangerous going forward, and overall they deserved to win, as simple as that."
The Championship side, who are without a permanent manager after Mark Warburton's sacking on New Year's Eve, took the lead through Eric Lichaj's header after 20 minutes. The American put them ahead again before half-time with a superb volley after Per Mertesacker had cancelled out his opener.
In the second half, Forest struck twice from the penalty spot through Ben Brereton and Kieran Dowell either side of a goal from Danny Welbeck.
It is among Wenger's worst defeats in the competition from his 21-year tenure at the club, per the Mirror's John Cross:
The Frenchman added: "It hurts very much. We have won the FA Cup three times in the last four years. We will not win it every year, and it's a difficult competition to win, but it hurts to go out in the third round. Unfortunately, they were better than us."
Football.London's Charles Watts agreed with Wenger's assessment of his team:
The manager made nine changes to the side that drew 2-2 with Chelsea in their previous outing, and the Gunners face the Blues again on Wednesday in the first leg of the Carabao Cup semi-final.
Welbeck, Mertesacker and Theo Walcott were among the handful of senior players included in the starting lineup, while Calum Chambers was the most experienced figure on a youthful bench.
James Benge of the Evening Standard said the lineup should have been capable of coming out on top:
James Benge @jamesbenge
Those that claim Wenger didn’t take the FA Cup seriously are wilfully blind to his record of doing exactly that. He may have got his tactics wrong but most players there are good enough to overcome any Championship side. These ad hominem attacks do nothing to solve Arsenal flaws
It's understandable Wenger wanted to give his stars a rest after a gruelling festive period, and the side he named included enough talent to win the game on another day—though it was perhaps a misstep to provide little first-team experience among the substitutes.
"We have a very difficult schedule, and many players were on the edge," Wenger said. "I understand the selection will be questioned, but we had eight or nine experienced internationals on the pitch and, with all respect, playing against a Championship team that would be the wrong excuse."
The club's outstanding recent record in the FA Cup has been their only saving grace in recent years, as they have routinely disappointed in both the Premier League and in European competition. But it will not be their salvation this term.
As ESPN FC's Mattias Karen noted, their exit puts added pressure on the Gunners:
Arsenal's only chances of winning silverware this season lie in the Carabao Cup—which requires beating Chelsea over two legs before a final likely to be contested against Manchester City—and the UEFA Europa League.
The club must throw everything they have into those competitions, particularly the latter given its greater prestige and the promise of UEFA Champions League qualification for winning.
A top-four spot in the Premier League isn't beyond the Gunners' reach, but anything less than full commitment to the two cup competitions they remain in would be inexcusable.