1) Key players missing
Manchester United will be without Wayne Rooney through injury and Juan Mata through suspension after his sending off against West Bromwich Albion.
That will come as a massive boost to Bilic's plans. United still have Anthony Martial and other quality players but Rooney has a fantastic record against the Irons and Mata has been the one making them tick recently.
2) Europa League a priority?
Not only that but van Gaal is under extreme pressure himself to qualify for the Champions League, which of course is the reward on offer nowadays for the winners of the Europa League.
Therefore the Red Devils will undoubtedly prioritise that game over the FA Cup, which only offers a route back into next season's Europa League, an achievement United are likely to cement through league position anyway.
So West Ham will know it will be Manchester United's strongest team taking to the field on Thursday for a fierce match and either a much-changed or at least fatigued team will take the field on Sunday.
3) Squad strength
West Ham have suffered with injuries this season and many fans feel they would be even higher in the table had they not lost seven key first team stars all at the same time over the winter period.
But despite the glut of injuries, Bilic and his squad have coped admirably.
Now as we enter the business end of the season, Bilic is starting to get his strength in depth back with the likes of Winston Reid, Victor Moses and Enner Valencia all set to return while Manuel Lanzini, Andy Carroll and Diafra Sakho are also now fully fit.
West Ham currently top the Premier League form table and go into the game off the back of four wins on the bounce including three in a week over Sunderland, Tottenham Hotspur and Everton.
In fact Bilic's side have won five of their last six games, drawing the other.
Manchester United however go into the match off the back of a disappointing 1-0 defeat to West Brom just when they looked to have found some form of their own.
Although form goes out of the window in cup games, West Ham will be brimming with confidence and Old Trafford holds no fear after they were desperately unlucky not to win there in a profligate 0-0 draw earlier in the season.
5) They've got Payet
The last time West Ham won at Old Trafford in the FA Cup their mercurial, ridiculously talented forward got the winning goal, Paolo Di Canio.
Well Dimitri Payet is the best player the club has had since the brilliant Italian and on his day he can be unplayable.
The French star got the last minute winner in the dramatic 3-2 triumph over Everton last time out and will be a constant threat to the hosts.
Payet is a big player and he turns it on in the big games and they don't come much bigger than this for West Ham.