Former boss Steve McClaren stepped down at the City Ground last week following a poor start to the season.
A number of names have been linked with the role but Robertson, who has served as O'Neill's assistant at each of his previous five clubs, has said that the East Midlands side should at least ask the Northern Irishman if he is interested.
"My only opinion would be if they want him, give him a call and put the onus on him and see what he says," he said. "If he doesn't want it then move on and go for the next one."
Robertson was part of the club's heyday of the 1970s and 80s.