Delays in Illinois Medicaid payments could force private residential-care facilities to evict hundreds of mentally retarded patients in the coming months, officials say. ``If nothing changes, we will see 50 to 1,500 people discharged from community residential centers throughout the state,'' said Peter Mule, director of the Riverside oundation, a home for retarded adults in Lincolnshire. Earlier this week Riverside received a reimbursement check from the state Department of Public Aid for December, but is still owed at least $250,000 for care provided since then, Mule said. Another non-profit agency, Clearbrook Center in suburban Rolling Meadows, will have to close within three months if $446,000 owed by the state can't be found elsewhere, center president Guerin Fischer said. Reimbursements are slow now, and they're bound to get worse, said Edward Duffy, state public aid director. ``I'm not choosing to do this, I'm running out of money,'' he said. Duffy said because the General Assembly underfunded the Medicaid program by $159 million last year, reimbursement money will run out. Requests received after April 15 won't be paid until July, Duffy said. ``We're working along with banks to help these agencies get credit lines,'' Duffy said. ``But the only real solution is more money.'' If the private homes are forced to evict their residents, the state would have to make room for them in already crowded facilities. ``We would have to put these people up on cots in gymnasiums,'' said David Devane, a spokesman for the Department of Mental Health. ``They would be without programs, without any treatment that is not life sustaining.'' Gov. James R. Thompson's proposed $22.2 billion budget for fiscal 1989 includes a $342 million increase for the Public Aid Department to pay past-due bills and reduce the delays in Medicaid reimbursements.