Jabil, a major manufacturer of electronics components for the tech industry, made a startling discovery in the spring of 2021: It had been relying on dozens of undocumented workers, contracted from a staffing agency, to power its manufacturing sites near Silicon Valley.

Jabil insisted they be fired, setting off what it called a “mass exodus” from its work force that required expensive and “herculean efforts” to find replacements, including hosting job fairs and borrowing workers from a client.

The upheaval caused the company to fall behind on both existing orders and bids for new business, costing it up to $50 million, according to interviews and allegations in an ongoing lawsuit against the staffing agency.

How Jabil navigated the sudden loss of undocumented workers — years before Donald J. Trump won re-election on a pledge of mass deportations of illegal immigrants — foreshadows the possible road ahead for companies that rely on staffing agencies to fill jobs at factories, warehouses and distribution centers.

The New York Times reported in November that staffing firms were among the top employers of unauthorized workers at work sites inspected for immigration violations over the past decade. Now with Mr. Trump’s victory, some firms are fearing the worst.

Toby Malara, vice president for government relations at the American Staffing Association, a trade group, said in an interview that the “business community at large” has been urging the incoming Trump administration “not to go forward with the mass deportations.”