The other big surprise was benching Aaron Ramsey and starting the rather awful Granit Xhaka in his place. We have some live text updates from the game, along with some video highlights after the match is over!
Pierre-Emerick Aubameyang was lured to the Emirates Stadium by former Arsenal manager Arsene Wenger during the January transfer window for a club-record fee, which is reported to be £60m.
City, who won their first two games of the season in fine style, again largely controlled affairs against Championship winners Wolves at Molineux but still had to come from behind to earn a point against Nuno Espirito Santo's impressive side.
Welbeck came off the bench to gleefully blast in from close range, ensuring Manuel Pellegrini's fledgeling West Ham tenure stands at three defeats out of three.
While the east Londoners are still looking to get off the mark this season, Pellegrini is seeing signs of life at the club.
The contract is another thing for his agent and the club, but we want - and I need - his focus only on training, on the match and his performance each day.
But, while Emery's team were dismal in defence, they possessed just enough attacking quality to stage a much-needed fightback.
"Today, he was progressing like the first two weeks, very positive".
Arsenal are aiming to at least challenge for the Premier League title this season after they ended up in sixth place in the table and without a trophy last term. I'd like to say he could have done more on the one goal he let in, but if he didn't see it, he didn't see it.
"It's clear we need to improve and we need to continue working to not concede as many chances as today", the Spaniard said.