Ajax, Celtic, Benfica and Porto went out but Manchester United hardly broke sweat after Simon Deli of Brugge conceded a penalty and earned a red card in the 23rd minute at Old Trafford and won 5-0.

Arsenal had won the first leg of the round of 32 tie 1-0 in Greece.

Defender Pape Abou Cisse cancelled out that advantage when he headed in a corner after 53 minutes.

Arsenal could not break through and were forced to extra time.

Giorgos Masouras hit the bar for Olympiakos in the 112th minute and a minute later Pierre-Emerick Aubameyang struck for Arsenal, putting them back in front with a spectacular flying kick.

However, in the final minute of extra time, Arsenal cleared an Olympiakos corner but were still disorganised when the ball was swung back into the area.

The cross found El Arabi free in front of goal for an unchallenged shot.

“Very hard, very tough,” said Aubameyang. “Their last-minute goal was unlucky for us.”

Last year Arsenal reached the final.

“It hurts a lot,” said Mikel Arteta, the Arsenal manager. “Attack against defence in 25 metres is difficult, but we created enough chances to win the game comfortably.”

Olympiakos coach Pedro Martins was delighted.

“We were determined, we believed we could qualify and we made it,” said he said. “Tactical intelligence, effectiveness, inner strength, courage…they all weighed in this success.”

At Old Trafford, one of United’s recent signings Bruno Fernandes gave the hosts the lead with a penalty after Deli’s flying intervention.

The other new boy, Odion Ighalo, making his first start, hit his first goal for the club in the 34th minute.