Arsenal manager Arsene Wenger says Per Mertesacker's red card against Chelsea was harsh.
The Gunners surrendered their top spot in the Premier League after losing to the Blues by 1-0 at the Emirates.
But the 66-year-old was still satisfied with the performance of his players.
"The referee was very quick to take the red card out," Wenger is quoted as saying by BBC Sport.
"We coped with the decision well and should have had a draw at least. We had the chances to get a draw. Mentally we were focused. Sometimes when you lose you can still say 'well done' to the players.
"I can't influence the decision of the referee.
"I thought it was harsh and we had to cope with it. I don't know what to say. It is a few games now that has happened. All we can do is keep our focus and fight. Yes [on Costa making the most of the situation], that is the game of the striker. Diego Costa is good at that."
"It is not a coincidence that after 23 games Leicester are still there. But there is a long way to go. What we take from today is that we are disappointed but have a lot of positives from our attitude and spirit."
Asked why he took off striker Olivier Giroud straight after Mertesacker's red, the Frenchman said: "The fact is we needed to get strength on the counter-attack and needed pace.
"That is why I made the decision."