People do not fully understand how a new super hospital will affect other services in the area, an AM has said.
The £350m Grange University Hospital in Llanfrechfa, Torfaen, opens in spring 2021, treating patients who need serious, complex or critical care.
Monmouth AM Nick Ramsay said people were "unsure about what's going to be left" at Nevill Hall Hospital.
Dr Steve Edwards from Aneurin Bevan health board said communication would "be key" ahead of the opening date.
Newport's Royal Gwent and Nevill Hall in Abergavenny have been the two main hospitals serving the health board area since 1901 and 1969 respectively.
But when the Grange hospital opens, it will take responsibility for most emergency or specialist services, as well as specialist care for children and babies.
A&E departments at Nevill Hall and the Royal Gwent in Newport will become minor injury units.
Mr Ramsay said: "At the moment, people know where they stand. They know if their loved one is ill or has a heart attack, for instance, they should come to Nevill Hall. That's going to change when they open the critical care centre in Cwmbran.
"This hospital has been providing services for the local community for a very long time and people are worried, they're unsure about what's going to be left here in the future."
The health board - which serves almost 600,000 people - said 75% of people would still receive care in the same way once the new hospital opens.
A Welsh Government spokesman said: "The Grange Hospital will bring together complex and more acute services on to one site and improve the quality of care for the very sickest patients.
"The health board is responsible for communicating changes to the local community."
Dr Edwards, the health board's deputy medical director for secondary care, said: "It's important for people to realise that they will still be able to access minor injuries and still have medical assessments, and the vast majority of care that's delivered within the health board will still be within those hospitals.
"As we move towards opening the hospital, communication with our population so that they understand where they access those services is going to be key."
The hospital is set over 55,000 sq m and will have 470 ward beds - large parts were preassembled at contractor Laing O'Rourke's site in Worksop, Nottinghamshire, and driven to Cwmbran.
Mike Lewis, project director, said: "All the bathrooms, we build them off site. They're built in our factories. They come complete, they're delivered, they're lifted into position.
"We build the bedroom around the bathrooms, that's the first thing that we put in. It's amazing to think that only two years ago, this was a greenfield site and it's only 15 months that we came out of the ground."