Bilic replaced Sam Allardyce at Upton Park on June 9 but will only be in the stands for Thursday’s clash with Andorran minnows FC Lusitans, despite a sell-out crowd packing the stadium.
The Hammers reached Europe through the Fair Play League and so enter the competition at the first qualifying round, just 40 days since their final fixture of last season at Newcastle.
Former Luton and Derby boss Westley has already named his team for the first leg, with first-team regulars James Tomkins, Morgan Amalfitano, Matt Jarvis and Diafra Sakho all starting.
And the 55-year-old explained why he was being entrusted to guide West Ham to a comfortable first-leg lead to defend in Andorra next week, rather than the game providing Bilic with an easy-looking start to life in the Upton Park dugout.
“You find you’re in Europe so someone has to put a plan together,” he said.
“We’ve had to be proactive to put a plan together very quickly as we didn’t know whether we would be in Europe until after the last game.
‘We knew what the group would consist of. It was to make a smooth transition for when the manager was put in place.”
Westley has been working with the squad available for Thursday’s game for the last 10 days, with the likes of highly-rated youngsters Reece Burke, Reece Oxford and Lewis Page joining up with some senior players who missed the end of the campaign through injury.
Meanwhile, Bilic has been casting an eye over the remainder of his squad in Ireland but, with three more qualifying rounds to come if West Ham are to reach the group stages, Westley knows the squad for the early European games will probably remain much the same.
“Blooding young players is crucial. Lewis and Reece (Oxford) haven’t played for the first team yet. We’ve got other players on the bench too,” he added.
“The cohesion of the team has been first class. At some point there will be a decision made when this is a first-team game. It’s a balancing act who plays.
‘It’s worked quite well in terms of a group going over to Cork. Kevin Nolan, Aaron Cresswell and James Collins are with the manager preparing for the start of the season.”
The sell-out crowd will no doubt welcome former defender Bilic with open arms as their final season at Upton Park ahead of their move to the Olympic Stadium gets under way much sooner than initially anticipated