Modric has attracted the interest of Chelsea, Manchester United and clubs across Europe in recent times, and has seemingly made himself open to a potential transfer with comments he has made to the press.
But Redknapp is determined to keep his star midfielder - even if he admits the Croatian has had his 'head turned' by some of the speculation - and has been told by Spurs chairman Daniel Levy that the club will not allow him to be sold at any price.
"If clubs the size of Manchester United and Chelsea are prepared to offer him Champions League football and massive wages, you would have to be very naive to think it wouldn't turn Luka's head," he told the Sunday Mirror.
"But I have been assured by the chairman that he is not prepared to sell him under any circumstances.
"And it's fair to say he's not happy about the unwanted attention and continuing speculation surrounding Luka.
"I have said on many occasions he is irreplaceable and it would be a massive disaster for the club if he was allowed to leave."
Redknapp is hoping to add to his squad in the transfer window rather than see key players depart, but admits it looks set to be a highly competitive marketplace with all the other top clubs looking to spend.
"We need to be strengthening the squad not depleting it," he added. "In an ideal world I would hope to bring in at least three more players to give us a realistic chance of competing for a top-four place in the Premier League next season.
"[Manchester] United, Liverpool, Manchester City, Arsenal and Chelsea, who all finished above us, are going to strengthen their squads so we have to be realistic.
"If we want to compete with them we have to be active in the transfer market, but I'm not suggesting it's going to be easy. Quality players at the right price are not easy to acquire.
"We also need to trim the squad and sell a few players."