West Ham's brilliant start to the season has fallen flat of late.
In truth the margin of victory could and probably should have been more as Bilic suffered his first humiliation since his return to East London as boss.
It also stretched the club's winless run to three games following the draw with Everton and 2-0 defeat to Watford beforehand.
It is a far cry from West Ham' electric start to the season which saw them boast the best away record in the Premier League with wins at Arsenal, Liverpool, Manchester City and Crystal Palace.
West Ham were becoming one of the most feared sides in the top flight, but the defeat to Watford seems to have dissipated that.
Puzzled by contrasting performances
Now Sullivan has revealed in his latest column on the club's official website that he can't quite believe the contrast in performances.
"We know we need to do better in the coming weeks, starting against West Brom."
Unfortunately for West Ham their home form has been indifferent and West Brom go into the game off the back of a morale boosting 2-1 win over Arsenal.
Indifferent home form
The Hammers have lost to Leicester City and Bournemouth at Upton Park and have been held to draws by Norwich City and Everton.
The loss of star player Dimitri Payet and £12 million striker Enner Valencia to long term injuries has made things difficult for the Hammers.
But as Sullivan says they must find a new winning formula.
West Ham fans will be turning up expecting a victory on Sunday and Tony Pulis will know that and set his side up to frustrate them.
With a trip to Manchester United on the horizon after West Brom it adds even more importance to the game against the Baggies.