PCP Partners, backed by Saudi Public Investment Fund, are set to complete a £300 million takeover the club.
Benitez has been tipped to return and replace Steve Bruce at St. James' Park, but Allardyce thinks it would be unwise.
"It's going to be very difficult for Steve, I think he'd be worried if the takeover happens," Allardyce told talkSPORT .
"I've experienced new takeovers twice – once at Blackburn [with Venkys] and once at Newcastle with Mike Ashley.
"I wasn't Mike's man and I can understand that, and Steve must be very worried from that point of view.
"There was a lot of hoo-ha about whether Steve was good enough or not, but I think he has done a very good job in his first season, considering he came in just two weeks before the season started, and at the moment they are going to be a Premier League side next season.
"I still don't understand why Rafa Benitez was deemed to be so good. I just don't understand it because Newcastle played such negative football then, it was unbelievable.
"But there you go; Rafa did a great PR job in himself and how he managed that club at that time."
Allardyce continued: "Steve is a very, very good Premier League manager of a great high quality, irrespective of what the media might say, and he knows how to put a club in a position to move forward.
"If you change everything around, sack everybody and bring new people in, then it's going to take years and years and they may well waste their money spending it quickly on the wrong players."