Call Me Joe (1957) by Poul Anderson?
From Wikipedia:
Anglesey uses a wheelchair. He is bad-tempered, dislikes all his colleagues, and is disliked in return. He is allowed to stay on the station only because of his ability to establish a telepathic connection with and thereby control Joe, a creature designed to survive the hostile conditions on the Jovian surface. Cornelius conjectures that something in Anglesey's mind rejects or fears Jupiter, and the resulting feedback keeps destroying the delicate equipment.
Eventually Cornelius is allowed to share a session with Anglesey during an important part of the mission. A set of autonomous female Jovians, similar to Joe but lacking a human controller such as Anglesey, has been launched from the satellite and will soon land on Jupiter. Joe, still controlled by Anglesey, is to be the leader, and father, of a new race that will live on the planet. During this session, Cornelius becomes aware of a third mind – that of Joe himself. Anglesey's mind has been steadily transforming itself into Joe and shrinking in the process. Cornelius was looking at the problem from the wrong end – it was not Anglesey's fear of going to Jupiter and becoming sublimated into Joe's stronger character which was causing the blowouts, but his fear of leaving Jupiter and the freedom Joe's whole and healthy, though non-human, body allows him. Anglesey's existence is poor and constricted compared to Joe's, and the environment has shaped a personality that no longer wants to be human.
Seeing himself from Cornelius's perspective, Joe becomes fully self-aware. He ejects Cornelius from the loop and shuts down what is left of Anglesey.
Found with the Google query science fiction novel avatar ripoff which brought up, among others, this GeekTyrant article asking whether or not James Cameron's movie is a ripoff of the above.
