Arsene Wenger was scathing of Arsenal's performance as they launched their Champions League Group D campaign with a chastening 2-0 defeat at Borussia Dortmund.

Ciro Immobile and Pierre-Emerick Aubameyang struck either side of half-time but the German club would have won by a landslide if it were not for the reflexes of Wojciech Szczesny and their own wastefulness.

Arsenal’s defending was woeful and reached its nadir for Immobile’s opener, but Wenger admitted they had under-performed across the pitch. “We lost the ball at the throw in but it was still three against Immobile at the back,” Wenger said.

“It’s still difficult to understand how we concede the goal. There were still enough people to stop the goal.

“Dortmund were very strong in midfield. They were stronger than us in midfield.

“But it’s very difficult on the night to go into any individual assessment of any player because as a team we were not at the requested level.

“It was a difficult night. Dortmund were the better team. Our performance was disappointing.”

Three days after missing a guilt-edged chance against Manchester City's Joe Hart, new signing Danny Welbeck failed to covert a one-on-one with another goalkeeper.

This time Roman Weidenfeller was the beneficiary of his wayward finishing and crucially the chance came four minutes before Immobile's opener. Welbeck's record in front of goal has been criticised, but Wenger claimed his recent signing from Manchester United needs time to settle.

“The paradox of the night is that we had very good chances to score first, but they then scored from counter-attacks,” Wenger said.

“Danny will improve, I don’t think we have to make a problem of that. “He had a very lively first-half. He had two or three good chances but couldn’t finish them. There was one obvious one. We have to be patient with him.”

To complete a dispiriting night that has opened Arsenal's transfer policy and Wenger's team selection open to criticism, Jack Wilshere limped off with an ankle problem.

“Wilshere has turned his ankle. It’s difficult to say how bad it is because I’m being cautious,” Wenger said. “Normally it’s not very bad but because of his history (with ankle injuries) I’m being cautious.”

Dortmund had 22 shots on target while Arsenal could muster only four in a defeat far more emphatic than suggested by the scoreline. “We had a couple of other possibilities to score, but there were the great reactions of Wojciech Szczesny,”

Arsenal midfielder Mikel Arteta was visibly disappointed after the defeat. "We faced a tough side in one of the most difficult stadiums in Europe, we knew that from the start.

“They put us under pressure, it was very sloppy from us to concede the goal with 20 seconds to go in the first half from a long throw from the other side of the pitch and after you are 1-0 down, it’s very difficult. They are very quick on the break and we didn’t perform well enough tonight.”

The 32-year-old Spaniard was aware of the pace Dortmund had up front which Arsenal failed to cope with.

He added: “They put a lot of pace and a lot of pressure up front. When they play here and when you (are familiar) — we’ve been playing here every year in the Champions League — but we couldn’t cope with that and we gave the ball away too many times in our own half in difficult positions.”

For the second time in two matches Danny Welbeck illustrated why his finishing is considered below par for a top forward by failing to convert a one-on-one with a goalkeeper.

With the match goalless and Arsenal under relentless attack, Welbeck managed to drag Aaron Ramsey’s clever pass wide with just Roman Weidenfeller to beat.

It was a key moment and the recent signing from Manchester United, who had other chances to open his Gunners account, only enhanced concerns over his record in front of goal.

On the Gunners’ chances during the match Arteta added: “They took the game from where they wanted to take it and that’s the disappointing thing for us.

“I think we had three or four very clear chances and I think they had three as well very clear, when it came to chances it was probably equal but I think they were the better side tonight.”

Dortmund coach Jurgen Klopp was delighted with the result and said matches like this make his job one of the best in the world.

He said: “We had three players ruled out of the team to muscle problems so it was not the best night we had but in the morning we made some decisions and I have to say it was a great performance from my team. It was really strong.

“We were brave and fast and really worked, it was everything we needed for this game. If you give them one metre of space they are so strong and they make everything with you. You need to brave against these strong players.

“You need go out in space and look for one-to-one situations and be brave enough to lose it because the next one is next to you. I enjoyed this game from the first second to the last.”

Looking ahead to his side’s next tough encounter — Saturday’s return to the Bundesliga, Klopp added: “We did what we have to win this game and for us it was near perfection so the important thing for me is three days and against Mainz, my former club.

“It’s hard work all the time but if you see games like this, it’s the best job in the world.”