London - West Ham's record signing Ghanaian international Andre Ayew is to undergo a scan on the thigh he injured in his Premier League debut for them in the defeat by Chelsea.
The 26-year-old France-born forward - who cost West Ham £20.5m when they signed him earlier this month from rivals Swansea - cut an unhappy figure as he left the pitch midway through the first-half of the 2-1 defeat.
"Ayew has a thigh injury, he told me it's not a big one but you never know," said Bilic.
"He will have a scan and we have to wait for the result of the scan to know how long he will be out."
Even if he recovers Ayew is highly unlikely to feature in West Ham's Europa League first leg qualifying play-off clash in Romania on Thursday against Astra Giurgui.
French star playmaker Dimitri Payet could play but he is unlikely to start against the unheralded Romanians as Bilic believes he still hasn't got the legs after his Euro 2016 exertions to last 90 minutes.
"It was a plan for Dimitri to come on for not more than half an hour.
"I spoke to him yesterday, he felt good and he looks good in training, but he's not ready yet for 90 minutes. It was the plan to bring him on.
"He gives you composure, he creates opportunities and space for other players."