Arsene Wenger was at a loss to explain his team’s performance after they crashed to defeat in their Champions League opener against Borussia Dortmund.

Goals from Ciro Immobile and Pierre-Emerick Aubameyang either side of half-time sealed the points for Dortmund – who could have won the group D tie by a landslide.

But the German club squandered several opportunities and Arsenal were also indebted to the reflexes of goalkeeper Wojciech Szczesny, who was repeatedly exposed by poor defending.

Wenger, who handed teenage full-back Hector Bellerin his full debut in place of the injured Mathieu Debuchy, acknowledged that the Gunners had been below par right across the pitch.

“We lost the ball at a throw-in but it was still three against Immobile at the back,” said the Arsenal boss.

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

“Dortmund were very strong – they were stronger than us in midfield. But it’s very difficult 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 and Dortmund were the better team. Our performance was disappointing.”

Recent signing Danny Welbeck might have put Arsenal in front shortly before half-time, failing to convert a one-on-one opportunity against Dortmund goalkeeper Roman Weidenfeller.

Just four minutes later, Immobile burst past Laurent Koscielny to open the scoring – and Aubameyang made it 2-0 three minutes after the restart, rounding Szczesny after a slip by Per Mertesacker.

Aubameyang also rattled the crossbar soon afterwards and the Gunners rarely looked like fighting their way back into the tie, while Jack Wilshere limped off with an ankle problem to compound Wenger’s problems.

The Arsenal boss insisted that Welbeck, his £16m deadline day signing from Manchester United, needs time to settle at his new club.

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

“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. We have to be patient with him.”