Roy Keane says Manchester United cannot afford to let Cristiano Ronaldo leave the club without signing a replacement after claiming Anthony Martial is "not the answer".
Ronaldo has one year remaining on his contract at Old Trafford but is seeking a move away from United this summer to play Champions League football this season.
Former United players Gary Neville and Wayne Rooney believe United should allow Ronaldo to leave but Keane believes the Red Devils cannot afford to lose the Portugal international at the current moment in time.
Responding to suggestions Martial could replace the goals provided by Ronaldo last season, Keane said on Sky Sports: "You can't let Ronaldo go out the door and think Martial is going to be the answer, Martial isn't the answer.
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"He's had a decent pre-season? Big deal.
"They haven't got enough options at this moment in time. That might change in the next few weeks but from a footballing point of view, you just have to hold on to Ronaldo.
"You can't let him go when you haven't got the back-up behind it."
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Ronaldo started on the bench for United's opening match of the new Premier League season against Brighton on Sunday.
The match at Old Trafford was Erik ten Hag’s first competitive game since his appointment as United manager.
Ten Hag has been tasked with transforming the Red Devils into a side once again capable of challenging for major honours and Keane believes the Dutchman has a major rebuild on his hands.
"He's mentioned in a number of interviews about the culture of the club, which has to change," added Keane.
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"We've seen it in too many games last year, players not working hard enough, not running, not sprinting. He has to change that mindset because United do have a lot of talented players.
"But the stuff coming out about pre-season, a lot PR from United about how the players have embraced him, they're training on time, they're training properly.
"Praise the lord! That's what you're supposed to do."
Topics: Anthony Martial, Cristiano Ronaldo, Football, Manchester United, Premier League, Roy Keane