ACO Assignment Improvement Act of 2021 This bill establishes additional requirements for assigning Medicare fee-for-service beneficiaries to accountable care organizations (ACOs) under the Medicare shared savings program. Under current law, the program enables ACOs to receive payments for savings stemming from care coordination and management. The bill requires the basis for assignment to reflect beneficiaries' utilization of not only primary care services provided by ACO physicians, but also those provided by other ACO practitioners—specifically, physician assistants, nurse practitioners, and clinical nurse specialists.