Brayford was the Railwaymen's player of the season and has emerged as a target for Rams boss Nigel Clough, who managed the 21-year-old at Burton.
But Thordarson revealed Derby are not the only Championship side interested in signing Brayford.
He said: "We would only consider selling him if we received decent money for him. It would have to be a good offer to suit us and the player, because we could quite easily keep him and help him develop for another year.
"John has attracted interest from Derby but they are not the only ones. I am aware of two other clubs in the Championship being interested in him.
"That is healthy for us and I'm not surprised because John is a good player and will only get better. I think he could play in the Championship."