Mikel Arteta said there was no excuse for Nicolas Pepe's "unacceptable" red card and admitted Arsenal's lack of goals is a "concern" after they clung on for a 0-0 draw at Leeds United.
Winger Pepe was given his marching orders early in the second half at Elland Road on Sunday for headbutting Ezgjan Alioski off the ball.
Pepe's dismissal was the fifth red Arsenal have been shown in the Premier League since Arteta took charge in December 2019, at least two more than any other club.
Leeds struck the woodwork three times and Bernd Leno produced a string of superb saves as the Gunners held on for a point which leaves them 11th in the table.
Arteta was not impressed with Pepe, whose contact with Alioski saw him sent off once referee Anthony Taylor reviewed footage.
He told BBC Sport: "It is unacceptable. Unacceptable. At this level you cannot do it."
Arsenal have scored just one goal in their last five Premier League games and that was a Pierre-Emerick Aubameyang penalty in the win at Manchester United.
The north London club have found the back of the net just nine times in the top flight this season, which is their lowest tally after nine games of a league campaign since 1986.
Arteta, who started captain Aubameyang in a central role, knows they must offer more in the final third if they are to move up.
When asked about the lack of goals, the Spaniard replied: "It is a concern. You need goals to win matches. We need to create opportunities."
Pepe hit the crossbar with a cross but Arsenal registered just two shots on target, while Bukayo Saka was denied by Illan Meslier when he went clear late on.
Arteta said: "There were two parts in the first half where we were superior and dominant and created opportunities.
"The last 15-20 minutes we gave balls away in different positions and gave them space to attack. We were lucky not to go 1-0 down. The second half, playing this side with 10 men, is complicated."