West Ham co-chairman David Gold admits Aaron Cresswell could be facing a lengthy spell on the sidelines with a knee injury he sustained against Karlsruher on Saturday.
The 26-year-old left-back has been an almost ever-present for the east Londoners since his switch from Ipswich in 2014, missing just one Premier League match in the last two seasons.
But Cresswell suffered what appeared to be a serious injury in a clash with KSC midfielder Grischa Promel on the Premier League club's pre-season tour of Austria and he left the pitch in the closing stages after receiving treatment.
Hammers boss Slaven Bilic expressed concern for the Liverpool-born defender after his side's 3-0 win, saying: "The only downside of this game is that we have to see what is [wrong] with Cresswell.
"He got injured and it didn't look that good. We will have to assess it."
Fan site Claret and Hugh suggested that Cresswell could be ruled out for six months, and Gold believes the assessment is correct, although he is waiting on scans to determine the extent of the problem.
"CandH sources are accurate and we are apprehensive but it doesn't look minor. We await the scans in the next day or so good luck Aaron," he said on Twitter.