Parkes, who played at Loftus Road between 1970 and 1979, thinks that Redknapp would make an instant impact at the club.
"[Redknapp] is hovering in the background and he is a guy who will come in and make a difference straight away," Parkes, 62, told BBC Radio 5Live's Sportsweek programme.
"It is horrible when you see a manager in this position but I would think he would slowly be running out of time and I am not sure how long he can carry on in the job.
"You may have to bite the bullet and say it is time for Mark Hughes to move on."
Rangers' next two matches are away trips to Manchester United and Sunderland.