Arsene Wenger conceded Arsenal's stuttering Premier League title bid had suffered a "major setback" after they lost ground at the top with a shock 1-0 defeat at Stoke.
Jon Walters' second-half penalty handed Stoke a famous win at the Britannia Stadium on Saturday and left the Gunners four points behind Premier League leaders Chelsea in third place.
The north Londoners have now won just once in their last eight trips to Stoke and Wenger admitted it was the worst possible start to what he had branded a season-defining month.
"It is not slightly worrying, it is a big worry for us to lose a game like that. It's a massive setback," Wenger said.
"In a game like that, we didn't produce the performance we wanted. To win a title when you are expected to perform you have to perform. It's as simple as that."
Although it was Mark Hughes, as opposed to predecessor Tony Pulis in the Potters' home dugout, the game retained the same feisty edge so memorable from these teams' recent meetings.
Glenn Whelan and Charlie Adam both escaped without sanction from referee Mike Jones despite controversial challenges on Olivier Giroud, much to the French striker's frustration.
"I have nothing to say about that. I am long enough in the game to make my own judgement," Wenger said.
Potters boss Mark Hughes said of the incidents: "It would be a shame if people highlight that as it wasn't that type of game."
However, the Arsenal manager was unimpressed by Jones' decision to point to the spot after adjudging Laurent Koscielny handled inside the penalty area as Walters attempted to lift the ball over him.
"We conceded a goal that was a nice gift from the referee but that can happen," Wenger added.
"He was one yard away. How could he take his hand away in time? There was no intention at all."
Wenger's side could have few complaints with the outcome, however, after serving up a sub-standard display which left big question marks hanging over their seemingly faltering title challenge.
"We had a good defensive performance and overall we are unlucky to lose this game but offensively we did not create enough," Wenger said.
"Our offensive game was poor considering our standards. You have to give credit to Stoke, they defended well, with resilience.
"They made the game very physical but we knew that before the game. They have the highest number of fouls in the Premier League.
"We didn't cope well enough with it and at the end of the day we were punished."
The win continued Stoke's excellent record at home against the Premier League's big sides as they made the Gunners their latest scalp, having previously beaten Manchester United and Chelsea and drawn with Manchester City this season.
There is now a six-point cushion between them and the relegation zone and Hughes said: "The stars were aligned and it came together.
"We were excellent on the day to be perfectly honest.
"I've been made aware that Arsenal felt it wasn't a just penalty but I'm a bit bemused by that as it was clear-cut to most people.
"Regardless of that we deserved the victory by virtue of what we produced and our general play."
Having missed four of his previous six penalties, Republic of Ireland international Walters showed nerves of steel to slot home with 14 minutes remaining.
"That is what you get from Jon," Hughes said. "He is mentally very strong and is a senior player in the group.
"He takes responsibility every day so it didn't surprise me that he grabbed the ball, he wasn't going to give it to anyone else.
"I was really pleased for him that he was able to dispatch it as it was a pressure kick."