Porteous has a contract until the end of the season at Hibernian and he will not be signing a new deal with the club.
Hibs are prepared to sell him in the winter window rather than watch the 23-year-old centre-back leave on a free transfer.
Rangers are interested in the defender and some have tipped him to move to Ibrox in next month’s window.
Ferguson insisted that there are no doubts about Porteous’ ability and feels he has the potential to be a top defender.
However, the Rangers legend feels his temperament often lets him down and he is not sure that a better option than the defenders Rangers have in their squad.
Ferguson said on the Go Radio Football Show: “I have always said that he has got the potential but there is always that question mark about his temperament.
“That’s the thing that lets him down.
“No doubt that he is a good player but is he as good as Connor Goldson, Ben Davies, and Leon King?
“I don’t know.”
Porteous did manage to score against Rangers in Hibernian’s 3-2 defeat at Ibrox last week, as he further caught the eye.
