The transfer comes just six months after the Austria star almost sealed a move to the Far East.
The Hammers have been left with plenty of time to find a replacement.
But who could they target? talkSPORT.com has identified five potential replacements for the forward…
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Moise Kean (Juventus)
Recent reports suggested Everton were interested in signing Juve youngster Moise Kean – but if there is any sense at the London Stadium, West Ham will make a move for the striker.
Kean, who was the first player born in 2000 to debut in Serie A, would have hugely benefited by playing with Cristiano Ronaldo last season.
The teenager netted a solid six goals in his 13 Serie A appearances last season.
However, following the return of Gonazalo Higuain to the Old Lady’s squad and Paulo Dybala and Mario Mandzukic available, it is difficult see Kean getting much game time with the Italian champions next season.
West Ham should at least attempt to sign him on loan, with an option to sign him permanently at the end of the campaign.
Maxi Gomez (Celta Vigo)
The Hammers were strongly linked with a move for Gomez in January after Arnautovic admitted he wanted a move away.
The 22-year-old Uruguay international has been linked with a host of suitors after scoring 13 goals and providing five assists in 35 LaLiga games last season.
The likes of Liverpool, Chelsea and Aston Villa were said to be chasing the youngster this summer, but, according to Sky Sports News, the Hammers are still leading the race for his signature.
And reports on Monday claim West Ham have activated Gomez’s £44.85million release clause following Arnautovic’s departure.
He may soon become the club’s fourth arrival, following the captures of Villarreal attacker Pablo Fornals and goalkeepers David Martin and Roberto Gago.
Moussa Dembele (Lyon)
There was a lot pressure on the Frenchman’s shoulders when he moved to Lyon from Celtic last summer, but he delivered with 21 goals in all competitions for the Ligue 1 side.
The 22-year-old is no stranger to English football, having played for Fulham before joining Celtic.
But he was very young during his spell with the Cottagers and now he has more experience under his belt in the Scottish and French leagues.
He could prove to be a big hit in the English top flight, with his skill set seemingly perfectly suited to the division.
A move for Dembele would no doubt be a risk, but it may be worth taking as he would be a cheaper option than other forwards on this list.
Nicolas Pepe (Lille)
There is no doubting it would take a minor miracle for Hammers chiefs to pull off this signing.
Pepe has been the subject of heavy transfer speculation throughout the summer, with Bayern, Arsenal and Liverpool all showing strong interest in his services.
However, a deal is yet to be agreed which means the interested parties are having doubts about whether to pay Lille’s desired fee.
West Ham should swoop in on the 22-year-old, who netted 22 times and provided 11 assists in a superb campaign and finished as the Ligue 1’s second top scorer.
Wissam Ben Yedder (Sevilla)
Although he has been strongly linked with Man United, West Ham should attempt to sign Ben Yedder from Sevilla.
He single-handedly knocked United out of the Champions League with a superb display and he may want a fresh challenge in the Premier League with an up-and-coming Hammers side led by Manuel Pellegrini.
The striker bagged 30 goals and 11 assists in 54 games for Sevilla last season and would be one of the signings of the summer if he secured a move to the claret and blue outfit.
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