Arsenal manager Arsene Wenger says midfielder Francis Coquelin made a mistake when he was sent off during Saturday's North London derby.
The Frenchman was shown a second yellow card after committing a foul on Harry Kane with the hosts restoring parity minutes later.
And Wenger believes the Gunners would have taken all three points had they completed the match with 11 players.
"I feel we have [showed character]," Wenger told reporters.
"We showed good attitude and character and I'm happy and proud of our performance and attitude. We refused to lose the game. We have as well big regrets because I could not see how 11 against 11 could have dropped points today.
"We made a mistake at 1-0 when it was 11 against 11 but even though we had a shock loss and it went to 2-1, we refused to lose the game. We always looked dangerous going forward.
"I didn't have to say a lot to him [Francis Coquelin] after that tackle.
"He made a mistake and he knows he made a big mistake. At half time we warned him as well and it was a mistake. He wanted to do well and mistimed his tackle and got sent off. You cannot complain about it. I can complain that Dier was not sent off. I couldn't understand that.
"[But] let down is a big word.
"He made a mistake and he has to quickly learn from that because at a top level it is full commitment and 100 per cent in all the duels in a controlled way. He got a yellow card because he made a mistake to go down on the tackle."