Bristol City chairman Colin Sexstone says it is unlikely they will appoint a new manager by the weekend.
"It's quite possible that we'll go into Saturday with the current coaches, with Steve acting as manager," Sexstone told BBC Radio Bristol.
"We need to do this thoroughly, which I think fans would expect us to do."
Millen departed Ashton Gate two days after their 5-0 defeat by Blackpool, with the Robins bottom of the Championship table with just one win all season.
They have not played a game since then because of the international break but face Peterborough at home on Saturday.
Former Barnsley manager Mark Robins, ex-Cardiff boss Dave Jones and Sean O'Driscoll, who was sacked by Doncaster last month, have all been linked with the vacant position, and St Johnstone said they would not stop Derek McInnes from joining the Championship side.
While Sexstone would not be drawn on naming potential candidates, he has ruled out the possibility of appointing ex-West Ham boss Gianfranco Zola and City goalkeeper David James, who were both rumoured to be interested.
"We've had over 60 applications now and had a thorough look at them all. We've now interviewed five people who all brought different skills and there's still a few applications coming in now," said Sexstone.
"The majority are out of work but that's not a pre-requisite by any means.
"I've seen Zola's name attached but I don't think he's applied and David James didn't apply. I haven't looked too much at the speculation as there's all sorts of names about."
He added: "We are going as quickly as we can but it's far more important that we get this right. We don't want any knee-jerk reactions and want to make sure we've looked at all possibilities.
"This is for the long term, not a quick fix."