Joao Cancelo has a goal already and wants another. As we saw so often at Manchester City, he's storming up from full-back and a real threat and tries his luck from 20 yards but can't keep the strike down.

Image source, EPA

Theo Walcott
Former Arsenal and Southampton forward on BBC Radio 5 Live

That attempt from Evanilson was just a warning sign to make sure that the Arsenal players are switched on.

David Raya produced a good save, showing he is alert, but it is just a sign of what they can do.

Image source, Getty Images

Phil McNulty
BBC Sport chief football writer at Emirates Stadium

Arsenal have been given a couple of warning shots by Porto, with Evanilson the threat so far. The Gunners have been putting together some attacking moves but the visitors are looking to hit back.

All very tense so far.

Napoli's Matteo Politano swings in a first-time cross from the left flank, but Barca keeper Marc-Andre ter Stegen does well to gather it, with Victor Osimhen ready to pounce.

Alex Howell
BBC Sport at Emirates Stadium

Arsenal have been the dominant team but when Porto have gone forward you can feel the tension from the crowd. They know how important it is for Arsenal to get the first goal in this match.

What an effort!

Porto right-back João Mário pulls back a great pass for Evanilson. He takes one touch then spins and thumps a powerful effort at goal.

David Raya reacts to push it away and he needed to.

Matthew Upson
Former England defender on BBC Radio 5 Live

Image source, Getty Images

There was one Porto player that wasn't quite on the same page as the rest there, it may well have been Evanilson that didn't push up on time.

Arsenal were just about offside but it would have been close.

Pepe was trying to get his side up, not get dragged back into the box by Arsenal. They kept there nerve there on the set piece... Just.

Napoli's Victor Osimhen thinks he is through on goal and shoots across goal, but is denied by a foot of keeper Marc-Andre ter Stegen.

Osimhen is narrowly offside and it's a risky game Barca are playing at the back, but they are getting away with it at the moment.

Image source, Getty Images

It's good defending from Porto as Arsenal try a sneaky free-kick, with four players standing offside, who then attempt to come back on when Declan Rice whips it in.

Pepe has different ideas and orders his defensive line to push up, meaning the Arsenal players are all offside when Gabriel makes contact and heads it over the bar anyway.

Image source, Getty Images

Evanilson catches Jorginho with a late challenge and it's a free-kick inside Porto's half for Arsenal.

There's no pressure at all on Arsenal's centre-backs but there's a confusion when the long ball is played and William Saliba heads it.

It's a poor decision, landing straight at the feet of Evanilson.

He strikes wide but that was a warning that Arsenal cannot afford any sloppiness.

Jakub Kiwior links up with Leandro Trossard and he puts in a teasing cross for Bukayo Saka.

Wendell puts him off in the end but Saka will be disappointed as he gets the header all wrong. He had made a good run.

Fermin Lopez with a great chance for Barcelona. He makes a diagonal run from right to centre to chase on to a lovely ball forwards and the run is timed perfectly and he's well onside.

He only has Alex Meret to beat and lifts the ball over Napoli's number one from the edge of the penalty area, but puts too much on it and it doesn't come down in time.

Matthew Upson
Former England defender on BBC Radio 5 Live

Image source, Reuters

Porto have stopped Arsenal getting into that rhythm that they like to get into early on here, where they smother the opposition.

They have taken their time on throw-ins and set-pieces and Pepe took about 40 minutes stalling on a pass a few minutes ago.

Arsenal haven't really forced the issue yet though.

Bukayo Saka does so well, beating Nico González and poking a good effort towards goal.

It's fumbled by the goalkeeper, Saka wins it back immediately and Martin Odegaard almost squeezes through with a one-two.

From the resulting corner, Odegaard shoots just wide.

Image source, Getty Images

Declan Rice sends one down the line for Leandro Trossard to chase but the veteran centre-back Pepe comes across to deal with it comfortably.

Martin Odegaard wins it back for Arsenal on the right but his cross is headed away.

It's only half-cleared though, and Kai Havertz drills it back across the area before Porto eventually clear their lines.