Manchester United, Manchester City and Chelsea are believed to be keen on signing the Spurs playmaker, who has refused to rule out a possible departure from White Hart Lane.
Redknapp, though, maintains that Tottenham cannot afford to let Modric depart this summer.
"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.
"We need to be strengthening the squad not depleting it. 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.
"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."