The move instantly divided opinion with excitement at the prospect of his obvious talent matched by scepticism over his attitude and consistency.
"The English press liked to talk about Mario - but I promise you Marko will make me looking like the most boring guy in the world," Balotelli said back in 2013.
Arnautovic and Balotelli joke during a training session while at Inter Milan
"He is one of the funniest and craziest people I have ever met - there will never be a boring moment when he is around. It is important to remember he is a very good player though. He has real strength and toughness about him - he will adapt very well to the English game."
Jose Mourinho claimed Arnautovic had the 'attitude of a child' during a year together at Inter Milan while former England boss Steve McClaren insisted he was the 'craziest player he had ever managed' while at FC Twente.
The maverick forward has a lengthy rap sheet and those who had their doubts about him were vindicated when, just 110 minutes into his West Ham career he was needlessly and recklessly sent off for elbowing Southampton defender Dale Stephens.
The Hammers have been through the mill since, losing that game 3-2 before a 3-0 defeat to Newcastle which cast serious doubts over his manager's job.
Mark Hughes insists Arnautovic has matured since becoming an established Premier League player and a father.
He has served his three match suspension and will return to West Ham's squad for the trip to West Bromwich Albion on Saturday.
Now it is up to him to prove people wrong and make a name for himself in East London.
And after all Hammers fans love a maverick, just ask Paolo Di Canio.