Randolph and Adrian
For both Darren Randolph and Adrian, it was a testing season under Bilic last time around. The former Besiktas and Croatia manager chopped and changed between the duo, remaining seemingly unclear about who is number one goalkeeper was.
With both making numerous errors throughout the campaign, signing Cillessen would remove that doubt and provide the West Ham defence with consistency at the back and allow defensive line to create a relationship with the Dutchman.
If Bilic can integrate that defensive stability into his side, then the Hammers will give themselves every chance of finishing in the top eight of the Premier League.