The Croatian has been at the centre of increased transfer speculation since Redknapp was sacked as manager earlier this month.
Asked if he thought 26-year-old Modric would remain at Spurs, Redknapp told Sky Sports: "I don't know. I wouldn't be sure about that.
"As we saw in the European Championship [Modric] can play in any team in the world, there's no doubt about that.
"It would be [a huge loss]. He is irreplaceable. But I suppose if they get massive money for him they can buy three or four players and improve the team in other areas."
Redknapp opened up about his departure, saying he would have loved to have stayed at the Premier League club but held no grudges against chairman Daniel Levy.
"Managing Tottenham was great, I loved every minute of it but it's only a job of work," the 65-year-old said.
"I would obviously loved to have stayed. Things were going well and there was no hint of me leaving.
"But it's up to the chairman and the owner. It is their club, if they feel they want to go down a different route."
Former Chelsea manager Andre Villas-Boas is the bookmakers' favourite to replace Redknapp at Tottenham.
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