David Moyes has insisted Said Benrahma has not failed a medical and says West Ham are still hopeful of completing a deal.
Sky Sports News reported earlier on Friday that there had been issues with the 25-year-old's medical ahead of his proposed move to the London Stadium.
It is understood the terms of the deal remain under discussion and Benrahma could join West Ham initially on loan, with the Hammers having an obligation to buy.
Moyes refused to elaborate on any further details on the medical, but remained optimistic that the transfer would be completed before Friday's 5pm deadline.
"He hasn't failed his medical and we are still hopeful that something can be done later today possibly, but I just can't guarantee that at this moment in time," the West Ham manager said at his news conference on Friday afternoon.
"It's one of those things that you don't talk about people's medicals because that's something that has to be done and it's private as it would be for anybody.
"So, I am not going to discuss his medical but we still have hope that we can get it done today.
"These things happen in football they happen all the time - but we'll see."
Benrahma would become Moyes' fourth major signing of the summer transfer window following the arrivals of Craig Dawson and Vladimir Coufal as well as the permanent addition of Tomas Soucek.
Benrahma scored 17 goals in 46 appearances for Brentford last season as they reached the Championship play-off final - suffering a 2-1 defeat to Fulham at Wembley.
The Algeria international began his career in France with Ligue 1 side Nice and made 18 appearances for the first team before moving to Brentford in July 2018.
Benrahma featured 94 times for Brentford, scoring 30 goals during his stint at the west London club.
A domestic transfer window between Premier League and EFL clubs remains open until October 16 - dubbed Deadline Day Two - but how does it work?
The international deadline - where English clubs could buy, sell and loan players from abroad - closed on October 5, but an agreement between Premier League and EFL teams means players can still be transferred in England.
Following consultation between the leagues - and because FIFA allows a summer transfer window to run for up to 12 weeks - Premier League clubs can still do business with EFL sides for a further 11 days in a bid to bolster their squads.
They can make loan or permanent signings, as well as selling, but there cannot be transfers between Premier League teams.
All EFL clubs can still do domestic business with any English team up until Deadline Day Two on Friday.
The domestic transfer window will run until 5pm on Friday, October 16.