The Hammers have made a flying start to their Premier League season, but they were outplayed by Watford in their last outing.
And Bilic has stressed to his players that another performance of such low quality will not be tolerated.
“It was a collective bad day against Watford,” the Croatian boss admitted. “We beat Chelsea and all last week I spent more time talking about how we should stay grounded but we definitely we didn’t have that five per cent in the red zone and if you are like that and unlucky then you have no chance.
“So I told the guys this is unacceptable. It happened and let’s at least make sure it does not happen again.”
West Ham have been brilliant over the campaign so far, but they were well short of their best at Vicarage Road – and deserved to lose the contest.
The Hammers didn’t show the attacking class they have become renowned for, and they were punished by the Hornets for making too many defensive errors.
The defeat brings the Hammers back down to earth, but they are likely to still feel satisfied with the way they have performed to this point of the season.
At the moment, West Ham are sat in sixth place in the Premier League table, and they face Everton in their next fixture as they endeavour to get their push for Europe back on track.