Mikel Arteta rues lack of aggression as Porto stun Arsenal in Champions League clash

But said that it would be "cruel" to judge their return to the knockout stages on the last-gasp goal that settled the contest.

The Gunners' hopes of reaching the quarter-finals for the first time in 14 years suffered a blow as Galeno's fine late strike saw Porto win 1-0 in the last 16 first-leg tie.

With the second leg at the Emirates Stadium on March 12, Arteta now knows his side must win on home soil to reach the business end of the Champions League on their long-awaited return.

Arsenal, more than anyone, know there are no easy games at this stage of Europe's elite club competition – having fallen at this hurdle seven years in a row under Arsene Wenger.

This is their first time back at this level since the last of those in 2017 and it proved much more challenging than the recent Premier League thrashings of West Ham and Burnley for a team inexperienced on these occasions – Kai Havertz the only player from the starting XI with any previous appearances in the Champions League knockout phases.

The vistiors failing to register a shot on target on a tough night at the Estadio do Dragao with Porto more than a match for the Gunners, frustrating the visitors for much of the evening and ultimately capitalising on some lapse defending to secure a late victory.

"Obviously I am very disappointed the way we gave the game away at the end," Arteta said.

"Not managing that situation well enough. You get punished in the Champions League. If you cannot win it, you don't lose it.

"We really dominated the game but we lacked purpose, especially in the first half. You need to have much more aggression, you need to break lines, to play forward and generate much more threat on that backline.

"We will learn from it. Now it is clear, it is half-time. If you want to be in the quarter-finals you have to beat your opponent and that will be the purpose and the plan."

Asked if his team were naive to fall to defeat in such a way, Arteta added: "Well it's only the last ball, so if in 94 minutes they haven't had any naivety other than that one.

"I think it's a bit cruel to judge it. But it's true that it has had a big impact on the result. A lot of other things they did for the first time here were very good. When you give the ball away three times in that area – you cannot do it."

Arteta also criticised the performance of referee Serdar Gozubuyuk, who awarded 36 fouls – the most in a Champions League game this season – and often spent time ahead of set-pieces speaking to players in the box.

"From set-pieces as well every time we touched somebody it seemed to be a foul before we even kicked the ball. But we will learn and do better," added the Spaniard.

Porto, captained by 40-year-old Pepe, appeared much more streetwise to the task at hand, leaving head coach Sergio Conceicao happy with the result.

He said: "The team understood perfectly the spaces they had to step into to condition our opponents and also what we had to do up front to hurt them.

"It was a good game, a Champions League game. Our opponents had more of the ball, but Porto were always more dangerous."

Read More: Late Galeno wonder goal seals first-leg victory for Porto over Arsenal (planetsport.com)