Ole Gunnar Solskjaer lamented Manchester United's lack of aggression after a third Premier League defeat of the season at Old Trafford.
Arsenal won at United for the first time since 2006 to inflict another home reverse on Solskjaer's side and leave them 15th after six games.
United retained the diamond formation that served them superbly against RB Leipzig in their 5-0 Champions League thrashing in midweek but the quartet of Scott McTominay, Fred, Paul Pogba and Bruno Fernandes flagged against Arsenal enforcers Mohamed Elneny and Thomas Partey.
Roy Keane, in attendance at Old Trafford as a pundit, claimed United lacked leaders during the loss and questioned Marcus Rashford's body language. Solskjaer accepted United did not turn up in the first 45.
"Well, in the first-half there were no tackles, no fighting, you have got to create an atmosphere yourself," Solskjaer said in response to a question about Keane's comments on Sky. "These games against Arsenal were always fierce, tackles flying. We didn't have the tempo or intensity for that, absolutely, and I was disappointed with the first-half performance.
"In the second-half we were better, unfortunately they got the penalty and we just couldn't produce the response.
"We prepared well, we prepared [the players] for a big game. Arsenal is always a big game, sometimes it's hard to pick up where you left off because that's not how it works in football.
"Everyone wanted to do well, of course, we just couldn't get going. That's why we have a big squad, a good squad and today we couldn't produce the performances that we've seen before. That's football for you. We play against very good players as well."
United have not won any of their last six Premier League fixtures at Old Trafford, dating back to July, and next host West Brom after the November internationals.
Arsenal, Tottenham and Crystal Palace have all won at United this season but Solskjaer struggles to distinguish home and away matches played behind closed doors.
"I think home or away, it doesn't really matter now," Solskjaer opined. "Some of the grounds or access to grounds, no fans in there, today they would've helped.
"It's the quality of opposition as well at times. You're playing against good teams here, marginal decisions, if you win or lose, and today we lost on a soft penalty."
