Speaking to reporters after the game, Arteta admitted that his side's performance was "not good enough".
"At this level, it's not good enough," the assistant boss said. "We lost many duels and you pay the price. We gave the ball away in the build-up in difficult circumstances and they took advantage. We were not consistent.
"It was [surprising] but they're not perfect. Sometimes they have good days and sometimes bad days. Sometimes they are silk and sometimes they are not. We felt under threat every time we lost the ball. I won't judge the players just because we lost. You have to judge many other things during the game and they know that already.
"The reality is that Pep [Guardiola] wasn't here and we lost the game. I have no idea if we'd win the game 5-0 if he was. I wouldn't like to use the [small] crowd as an excuse. We had this type of crowd before and played well. I'd like it full and everyone supporting the team like crazy. It didn't happen but it isn't an excuse."
The result leaves City bottom of Group F after the first of six group stage games.