In the 63rd minute, Arteta was handed a yellow for a foul on Valon Behrami. The Gunners midfielder would commit another foul, this one on Jose Callejon in the 76th minute to earn the second booking.
ESPN's Ian Darke wasn't a fan of the referee's decision.
Arteta's sending-off means he'll miss Arsenal's next match, which will be the first leg in the round of 16. Depending on the matchup in that round, not having a player like that for even one match can carry heavy significance.
The Gunners could have a poor first leg and fail to recover in the second, exiting the Champions League in the round of 16 for the fourth season in a row.
Things could be much worse for the club, though.
If this had happened last year, Arsenal could've been in major trouble. This year, it's still a problem, but not the crippling blow it could've been.
The addition of Mathieu Flamini has meant a lessened role for Arteta in general, as the Frenchman has become the primary ball-winner/distributor sitting in front of the defense.
Unless Arsene Wenger wants to put out a side similar to the one that faced Napoli, he'll be free to start Flamini and Aaron Ramsey in midfield—a partnership that has been so successful this season.
The importance of Arteta's absence will only truly be known when Arsenal learn of their opponent for the next round. You could only have seen him starting the match in the first place if the Gunners faced off against somebody like Barcelona or Bayern Munich, who press a lot and can strike quickly.
Otherwise, Arsenal will remain in a good position for the first leg.