On Monday, Boeing's union machinists voted to accept the company's contract offer and end a strike that stretched for more than seven weeks. As the troubled plane maker gets back to business, the deal couldn't have come at a more crucial time for suppliers—and it's possible it didn't come quickly enough for one key partner.

On Tuesday, Spirit AeroSystems raised flags when it said the company faced "substantial doubt" over its ability to continue operating.

The Boeing strike forced Spirit, which makes fuselages for Boeing's 737 Max aircraft, to burn through cash reserve

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