It's really shame Arsenal lose against Porto, Porto is not consistent in the domestic league, even though Arsenal is also inconsistent in the domestic league. But there's a big difference between Premier League and Primeira Liga, a top 10 Premier League can beat the champion in Primeira Liga.
It's not funny if Arteta sacrificing Champions League for Premier League.
The Champions League is very different from the respective local leagues and that is why FC Porto was able to beat Arsenal even though Arsenal's players are quite the favourites.
In that match, I saw that Porto's players were mentally much more prepared than Arsenal's players and they were always able to play more calmly even though Arsenal didn't stop attacking.
The defeat experienced by Arsenal will be something difficult, even though the second leg of the match will take place at the Emirates Stadium and if FC Porto can make Arsenal draw with a small goal margin then FC Porto's chances of qualifying will be even greater.