The Croatian boss says the opening-day Arsenal win may have actually hindered West Ham's start to the season.
It was a disappointing display at home again from the Hammers after last weekend’s 2-1 defeat to Leicester, a game in which they also conceded two goals early on.
It was individual mistakes this week that cost them, however, with defenders Aaron Cresswell and Carl Jenkinson both having a bad day at the office, the latter eventually seeing himself sent off, and Bilic has apologised to the fans, labelling the display unacceptable.
"It was a tough day, I want to apologise to the fans because it was not acceptable," he told reporters, as quoted by whufc.com.
"It is tough to talk about it because if you talk about the first half you could talk for ages. I am so disappointed and I am sure the lads are. I am talking about basics - determination, closing people down and putting them under pressure. They won the game because of that."
A change of mentality needed at West Ham
The new manager was also keen to emphasise that his side must change their mentality in future.
After the surprise but impressive opening day victory over Arsenal, coupled with a decent summer of recruitment, hopes were high about how the side might far this season.
However, back-to-back home defeats have quickly killed the euphoria of that opening win at the Emirates - and Bilic says victory has perhaps made his side complacent, despite his warnings to them that such a mentality would be 'suicidal.'
"We have to change the mentality and only we can change it," he added.
"Maybe after that game against Arsenal we were thinking we were going to cruise and that we only needed to turn up on the pitch and it was going to be OK, but I said it then and that is not just wrong, it is suicidal."