The big question at Emirates Stadium is can Porto score an away goal?
If they do manage to beat stand-in skipper Manuel Almunia, then a shock could be on the cards in North London.
Porto are a decent side and confidence in the Gunners has drained since Cesc Fabregas was ruled out with a hamstring strain.
But it's difficult to imagine Arsene Wenger's free-flowing side not scoring for 90 minutes at home.
And 1-0, of course, will be enough to take them through on the away-goals ruling.
So it looks as though Porto will have to find the net at least once and that could be their problem.
A 1-0 reverse at Chelsea earlier in the competition was no disgrace and, while the Portuguese side are better at home than away, they triumphed 1-0 at Apoel Nicosia and, most impressively, 3-0 at Atletico Madrid.
A 3-0 defeat at Sporting Lisbon exposed their frailties last month but there have been some big wins of late for Jesualdo Ferreira's side - five were put past Sporting in the cup and Braga at the Dragao.
So Hulk, Falcao and co know the way to goal.
History is against Arsenal.
They've gone out nine times out of the last 12 when they've trailed from the first leg in Europe and the last occasion was way back in 1978.
But Wenger's side are the darlings of many in the UK and the fact their price has weakened to 4/6 will see plenty of interest.
We're not convinced and think the draw at 14/5 or 1-1 at 7s with Sky Bet may be the way to go.
It is a tough game to call, though, and we'll keep stakes at a minimum.
