The 144-page regulation would require businesses that don't offer insurance to pay a penalty if more than 30 of their employees receive government help to buy coverage.
The proposed rule would also require coverage to be "affordable," or costing no more than 9.5 percent of a single worker's income.
On Friday, Graves criticized the regulation for "problematic" definitions of "affordable coverage," "minimum value" and "full-time employee."
He added that many aspects of compliance will burden small businesses that lack "a human resources department, a compliance officer, accountant or general counsel to handle them."
Geithner left his post last Friday. The new acting Treasury Secretary is Neal Wolin, who will lead the agency until chief Obama adviser Jack Lew is confirmed.