Just three weeks after signing a new contract at Spurs, Bale admitted on Monday that he could leave White Hart Lane one day should one of Europe's top clubs come calling.
Redknapp said: "If you are looking to build a club, you can't be selling Gareth as then (Luka) Modric would want to go and then someone else would want to go. Then Tottenham would end up where they were four or five years ago, finishing half way up the table each year."
He added: "I had it at West Ham. We let Rio (Ferdinand) and Frank (Lampard) go and the rest followed.
"I'm sure it would have to be an amazing figure if you ever sold him. But how do you replace a player like that? It's very difficult."
Bale will get the chance to showcase his talents in the Champions League once more on Tuesday evening when he takes to the field in Tottenham's quarter-final first leg clash against Real Madrid.
After seeing the way he tore apart Inter Milan in the group stages, his probable return from a hamstring injury is a massive boost for Redknapp's side. Spurs were given an additional lift with the news that William Gallas could make a surprise return from a knee complaint that prevented him from kicking a ball just four days ago.
Other injuries mean that the Londoners may not be able to fill their allocation of seven substitutes for Tuesday's match, but Redknapp insists his team of underdogs can still beat Mourinho's side and make it to the final.
"I don't feel any pressure. No-one expected us to be here at here start of the season and we have a big chance of winning," Redknapp said. "We are not just coming here to turn up. We want to keep progressing.
"The dream is still there for us, we have to make that dream comes true. We want to go all the way if we can. We know how difficult it's going to be but there is still a chance and we have to take it."
Source: PA
Source: PA