The Liverpool attacker has proven one of their most valuable assets over the last few seasons - overtaking Mohammed Salah in many regards.
Mane won African Footballer of the Year in 2019 and is widely regarded as one of the best in the world.
The Senegalese attacker had 14 goals in 26 Premier League matches so far this season before the league was suspended due to coronavirus.
Hislop spent five seasons at West Ham and was in goal when they lost to Liverpool on penalties in the 2006 FA Cup final.
The 51-year-old was asked about a rumoured bid worth £140m that was being weighed up by Real Madrid and whether Liverpool should accept it.
"I think you take [£140m]," Hislop told ESPN FC.
"And listen, if you pay £140m for Sadio Mane, I think it’s good business."
Hislop went on to speculate that Mane, 27, may feel it's the right time to take on a new challenge and that Real Madrid are often hard to turn down.
"I still feel that playing for Real Madrid, Real Madrid is still the biggest name in terms of who players want to play for in world football," Hislop added.
"I know this Real Madrid isn’t the Real Madrid of even two or three years ago, but I still feel it has that attraction.
"And if you’re Sadio Mane, as good as it’s been with Liverpool, they’ve won the Champions League, still don’t know what’s going to happen this season but let’s say you win the Premier League, and at your age late 20s Real Madrid comes calling, you understand why you’re prepared to walk away from Anfield."
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