Speaking on Sky Sports, Roy Keane said: "You can't let Cristiano Ronaldo go out the door and think Anthony Martial is going to be the answer, Martial isn't the answer.
"He's proven that. He's a had decent pre-season? Big deal. Manchester United haven't got enough options in their squad.
"From a football point of view, you've got to keep hold of Ronaldo. That might change over the next few weeks but at the moment they haven't got the back-up."
Ronaldo is openly pushing to leave Old Trafford before the August 31 deadline in order to join a Champions League club and preserve his goalscoring record.
With Martial absent through a hamstring issue against Brighton last week, Erik ten Hag was forced to start Christian Eriksen in the False 9 role — to no avail.
