Per Mertesacker hailed Arsenal's defensive solidity after a 2-0 win over Borussia Dortmund which took them through to the Champions League knockout stages.

Alexis Sanchez's second-half strike settled the Gunners' nerves and the defence were able to close the game out.

They had led since Yaya Sanogo's goal just over a minute in and Mertesacker told Sky Sports 1: "That was good but we had to play another 89 minutes - and over 95 minutes we played absolutely solid and that was key.

"It was a solid performance, especially defensively. We didn't give them too much, especially in the second half, we played absolutely solid at the back."

Manager Arsene Wenger agreed with his player's assessment, particularly in light of Saturday's disappointing defeat to rivals Manchester United.

"(We had) the focus to defend well together," he said. "We had a similar game to Saturday but we focused really well not to give any chances away."

Mertesacker added: "We've had a few disappointing results so today was a mental test, but we are strong enough and now we can continue.

"After Anderlecht and after Manchester United we dropped mentally a bit but we learnt from those disappointments and we are delighted with the performance today.

"Now Saturday, an early kick-off away (at West Brom), we have to prove (ourselves) because last season we dropped a lot of points. We are through to the knockout stages and now we have to focus on the Premier League."

Sanchez has won rave reviews since his summer arrival from Barcelona.

Asked what the Chilean brought to the team, Wenger said: "Unbelievable fighting spirit and his class as well, he's a mixture of talent and desire."

The one sour note was the loss of both Sanogo and Mikel Arteta to injuries, with the latter of particular concern as Wenger battles a growing casualty list.

"It doesn't look good, it's a calf," the Frenchman said.

"It's difficult to take on the chin at moment, you go in to many games and need everyone available."