Watford midfielder Imran Louza has been ruled out "for a few months" with a broken leg - an injury that will see him miss the World Cup with Morocco.
Louza played for Morocco at this year's Africa Cup of Nations but did not feature in their most recent friendlies after undergoing knee surgery in May.
"It's a big blow," said Bilic.
"Unfortunately he will be out for long. We are still waiting for another opinion but he will be out for a few months."
All of Louza's appearances for Watford this season have come under Bilic, with the midfielder regaining fitness following his summer surgery after former boss Rob Edwards was sacked in September.