West Ham have announced that Andre Ayew is facing "a significant rehabilitation period" as a result of the thigh injury sustained in Monday's Premier League match at Chelsea.
The club's £20million record signing lasted just 35 minutes of his debut at Stamford Bridge on Monday before limping off.
The club have not given a timeframe for Ayew's absence - but Jack Sullivan, the son of West Ham joint-chairman David Sullivan, earlier wrote on Twitter: ''We are devastated that Ayew will be out for about 4 months. He needs surgery."
The club's head of medical and sports science Stijn Vandenbroucke told the club's official website
"Andre Ayew injured his right thigh at Chelsea on Monday and underwent a scan on Wednesday afternoon.
"The medical team will now assess the results with a leading consultant before drawing any conclusions.
"My first thought and priority is with the player and his physical and mental well-being. He has suffered a significant injury and it is imperative that we do not rush to conclusions and we receive the correct diagnosis by consulting top specialists.
"At this time, it is too early to put a time frame on his potential return date but Andre is facing a significant rehabilitation period.
"The best thing we can do as a club is to support Andre through this difficult period. We will keep you updated throughout this process and I am sure he will come back stronger than ever.
"Everyone at West Ham United wishes Andre a speedy recovery and return to action."
While not specifying when the player will return, the tweet of Sullivan junior suggest he may be back just before Christmas.
It is a major blow for manager Slaven Bilic, who had spent all summer attempting to bolster his attack before finally landing Ayew last week.
The Hammers, who face Astra Giurgiu over two legs for a place in this season's Europa League are without another new signing, Sofiane Feghouli, through injury - as well as Aaron Cresswell and Manuel Lanzini.
Bilic is taking the Europa League seriously this season despite admitting it could affect his side's Premier League form.
The Hammers qualified through the Fair Play League last season and were knocked out by Astra in the qualifying round, when Bilic fielded a team of youngsters.
However, he has brought a full squad to Romania, including the likes of Mark Noble and Andy Carroll, for the first leg of the rematch.
''We will put out a strong team,'' he said. ''We want to get to the group stages. These are two very important games for West Ham United.
''We want to qualify. It's a reward for a good season last year and only a few clubs from the Premier League are privileged to play in Europe. We are two games from the group stages and we want to participate.
''All the talk all season is 'will you qualify for Europe', and then you do and the next question is 'why did you qualify?' Well, we have to adjust.
''We know it affects your chances in the next Premier League game but we want to try it. It's good for everyone, it's good for the players to play games against teams from different countries, they become better players.''
Astra beat the Hammers 2-1 in Giurgiu last season but are in some disarray this time around having lost several key players, while coach Marius Sumudica has been suspended for betting on matches.
The Romanians were initially in the Champions League but were knocked out in the qualifiers by FC Copenhagen.
Bilic added: ''They have many internationals, although they sold a few players. They have a team who have played together for some time.
''They played a good game against Copenhagen at home, drew 1-1, and what is crucial is that they are current champions of Romania.
''I'm Croatian, so I know how good teams from Romania can be. I'm expecting a tough game.''
In addition to the Hammers' injuries, new left-back Arthur Masuaku is cup-tied, while the influential Dimitri Payet did not travel.
However, Argentinian striker Jonathan Calleri has travelled with the squad and could be involved following his loan switch from from Uruguayan club Deportivo Maldonado.