The National Autistic Society also expressed concern.
NHS Grampian defended its autism services.
Mr McDonald said: "I understand many local adults with autism struggle profoundly with their condition every day, with limited prospects of diagnosis or support in their local area.
"NHS Grampian has dispensed with the services of an expert diagnostician expert. Autism is a complex condition, and concerns have been raised that this decision may result in increased difficulties for some adults to gain access to a diagnosis and post-diagnostic support services."'Best supported'
An NHS Grampian spokesman said: "Not all people with autism have a mental illness.
"Those who do will continue to be assessed and treated through their Community Mental Health Team.
"People with autism but no diagnosis of mental illness are best supported through mainstream services and are referred to the local authority."
He added: "We have in the past used the independent sector for the diagnosis of autism, but it is the view of our clinicians that these assessments can be carried out in-house.
"There are a small number of clients whose care is very complex and they will continue to receive support from the independent provider."