Arsenal manager Arsene Wenger believes Per Mertesacker's dismissal during the 1-0 defeat against Chelsea on Sunday (24 January) was "harsh" but refused to single out Diego Costa over his role in the referee decision. The manager, meanwhile, said that dropping Olivier Giroud after his side were reduced to ten men was the most "rational" decision because Arsenal needed the pace of the likes of Theo Walcott up front to try to turnaround the situation.

The German centre-back was sent off in the 18th minute for bringing down Costa when the striker was in on goal, with Wenger opting to replace Giroud and introduce Gabriel Paulista to rebuild his defence. But just five minutes later, the Spanish international scored the only goal of the game to leave the Gunners third in the table, with the same points as Manchester City but three behind Leicester City.

Asked about Mertesacker's red card, Wenger said: "I think it was quick and harsh, but I have to watch it again. What can I change? It doesn't change anything. The referee made the decision and we have to get on with it. I didn't speak to Per Mertesacker [about it]."

"We have 35 journalists here, I am sure you have all seen it again and analysed it and know much more about that situation than I do. I can't comment because I didn't know if Costa was offside or not or if Per touched him or not. I don't know. It was of course frustrating but it is a decision and we had to get on with our job, which we did."

But when asked whether Costa influenced the referee as he did in the previous derby when Gabriel was sent off, Wenger said: "You want me to say things I am not interested in. I said Costa got two of our players sent off in the last two games against Chelsea. Those are facts. Was the decision right or wrong, I don't know but those are facts. Without accusing him of anything."

Meanwhile, Wenger hailed his decision to replace Giroud for the Brazilian centre-back to compensate the fact that his side had only 10 men. "You want to make a poll for every decision to see who does what? I made the decision and for me it was quite normal. We had to go for long distances and we needed pace to go from one goal to the other. We knew we had to drop deeper and use pace to get forward."

Wenger, meanwhile, claimed that the French striker missed the Saturday training session but questioned whether Giroud was fit to play the full game, he said: "I think yes, but it was a bit in my mind if he gets injured, with his ankle, he was a doubt, he didn't practice yesterday and today we declared him fit."

"It was on my mind. But I had to change a player and I thought that was the most rational decision with all the factors."