Bale has been linked with an exit from Real Madrid recently due to the emergence of Isco in his position while he has been missing through injury.
The former Tottenham man started the Champions League Final win over Juventus on the bench, making his first appearance since the Clasico loss to Barcelona in April.
Bale has revealed he is happy at Real and does not want to leave, with United linked with a move for him.
But Hislop thinks he should go as he is not going to be first choice under Zinedine Zidane anymore.
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Shaka Hislop says Real Madrid should sell Gareth Bale to Man Utd if they come up with the cash
If you get Manchester United to come up with the cash, you sell Gareth Bale no question
Shaka Hislop on Real Madrid star Gareth Bale
"I don't think Gareth Bale wants to go but I think he needs to go at this point," the former West Ham goalkeeper said on ESPN FC.
"Given what we've seen from (Karim) Benzema and the whole question mark about Benzema or (Alvaro) Morata coming into this final, I thought Benzema was outstanding, as was everyone else for Real Madrid in the final.
"Benzema stays and he plays and in all honesty given what you saw of Isco, who flew a little bit under the radar but I thought was vital to Real Madrid, particularly in the early stages where he just sat in front of that back four.
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Shaka Hislop believes Gareth Bale should leave Real Madrid
"In the early stages of the second half he kind of floated out to Real's right.
"I can't see Gareth Bale playing both of those positions and be able to do as versatile a job as Isco.
"If you get Manchester United to come up with the cash, you sell him no question."