Modric was caught in a tug-of-war between Spurs and Chelsea last summer, with the Blues launching up to three bids for the Croatian - the largest of which was believed to be £40 million. Spurs managed to hold on to their star midfielder and he has rewarded them with a series of fine performances that have left Manchester City manager Roberto Mancini purring.
Speculation about Modric's future, and that of Gareth Bale, will only intensify if Tottenham's run of disappointing results continues and the club fail to qualify for next season's Champions League.
Redknapp is confident Bale will stay at White Hart Lane this summer despite Barcelona's overtures, but he is less certain about the commitment of Modric, who is yet to pen a new deal despite being promised one last summer.
"Last summer it was vital that we kept hold of Luka," Redknapp said. "I don't think we will have to do it with people like Gareth Bale this summer.
"Luka, it's different with him. I don't know what he would do. I would be lying if I said I was sure that he would stay. You don't know. But certainly I wouldn't see Gareth Bale wanting to leave here.
"At the moment he is happy here, a big fish here. He is still learning the game, a young boy, and this is where he needs to be. You would like to think Luka would still be here, but you never know."
Tottenham dropped out of the top four last week after suffering yet another defeat at the hands of struggling QPR. Redknapp is confident his team can win all four of their remaining games and leapfrog Newcastle back into fourth, but that still may not be enough to earn a place in the Champions League.
UEFA rules state a maximum of four teams from one country can qualify for the Champions League and, as a result, the team who finish fourth in the table will not qualify for Europe's top club competition if Chelsea beat Bayern Munich in next month's final in Germany.
"I think that (the rule) is unfair," Redknapp said.
"You look at the teams that some other countries have in it and we have a team in this country that might finish fourth and are in the final. That shows you the strength of our league.
"I think the rule should be whoever finishes in the top four should get a Champions League place. I don't see any reason why there can't be five English teams in it.
"I don't know whether they will change it, though. You know what it's like with people changing rules."
Redknapp concedes, however, his team are now outsiders in the battle for a top-four place.
"At the moment I suppose Chelsea might be favourites to finish fourth," he said. "We just have to finish fourth, or third if possible, you never know. It is possible."