The incident, which happened just after 2pm on Monday, involved a lorry driven by a 43-year-old man colliding with a 61-year-old who either stepped or stumbled onto the main northbound carriageway.
The 61-year-old had apparently been working on his white Volkswagen Passat on the hard shoulder of the northbound carriageway just before the collision.
He was taken by ambulance to Glasgow Royal Infirmary where his condition is described as serious but stable.
Police enquiries are continuing and Sergeant Stewart Dyer from the Road Policing Department based at Motherwell is asking any witnesses who have not yet spoken to police, or anyone with dash-cam footage to contact officers as soon as possible.
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Anyone with information should contact officers through 101 quoting incident number 1976 of March 11.