Arsenal manager Mikel Arteta railed against the “bizarre” decision to award Brighton a penalty in his team’s damaging draw on the south coast, saying he had never before seen an incident like it.

Arteta’s side were leading 1-0 at the Amex Stadium when Arsenal defender William Saliba caught João Pedro, the Brighton forward, in an unusual head-on-head collision inside the penalty box.

Replays later showed that Saliba appeared to make a slight touch on the ball with his head before making contact with Pedro. The officials, however, deemed that there was “sufficient contact” to award the penalty, which Pedro converted.

“I have never seen something like this in my life,” said Arteta. “Bizarre. I asked the boys if they had seen [something like] it and nobody has before.

“When you look at the incident, the distance, the player, João touching the ball and Saliba touching the ball, you can see contact there. So, it is a new one. We are really disappointed with the decision.”

Asked if he expected the penalty to be overturned by VAR, the Arsenal manager added: “I checked and after three seconds they said they already checked it. Seems quick.”

Fabian Hürzeler, the Brighton head coach, said it was a “clear penalty” and suggested that there would have been no complaints if there had been similar contact between Saliba and Pedro’s legs, rather than their heads.

“If it happens with a different piece of the body, for example knee on knee…” said Hürzeler. “It is just the fact that it [Saliba’s challenge] is too late.”

The penalty proved to be the decisive moment in the match as Arsenal dropped two more points in their pursuit of league leaders Liverpool. The gap between the two sides now stands at five points, but Liverpool have two games in hand on Arteta’s team.

Arsenal’s evening worsened with a muscular injury to teenager Ethan Nwaneri, who scored in the first half but was then substituted at the break.

Nwaneri was filling in for the injured Bukayo Saka and Raheem Sterling on the right wing but will now join those two players on the sidelines, further weakening Arteta’s attacking options. His absence for any meaningful length of time will also increase the clamour for Arsenal to invest in a new forward during this month’s transfer window.

“Really bad news because I think we’ve lost him,” said Arteta of Nwaneri. “We had to take him off at half-time with some muscular issues.”

Arsenal also came into the game without the ill Kai Havertz, their top scorer, and with key players Martin Odegaard and Gabriel Martinelli only fit enough to start on the bench.

“The run that we are in, especially with all the situations, illnesses and injuries, and players that we are missing, I think the team deserves a lot of credit,” said Arteta.

“We’re playing every three days. I understand how much we want to [win], but in terms of the quality and the consistency and doing the simple things right against a competitive team, we didn’t do enough today.

“We lacked control and gave the ball away and we didn’t do the simple things well enough to be really on top of the game and have the dominance we required.”

Such was Brighton’s improvement in the second half, Hürzeler was frustrated that his team did not claim all three points. “We are not clear enough with goalscoring chances,” he said. “We have to work hard on ourselves and look in the mirror and see that we are responsible for the result.”