The 29-year-old man was flown by air ambulance to Southampton Hospital in a critical condition after the collision, which happened on Tuesday, May 21, at 9.35am near Swandean Hospital.
The man has been named locally as Ryan Chapman and had been at Swandean Hospital.
The lorry driver suffered shock.
Police closed the A27, its junction with the A24 and the A280 for more than three hours for emergency services to deal with the incident and for the air ambulance to land.
Anyone with information who saw the collision is asked to call Sussex Police on 101 quoting Operation Daventry.