The 25-year-old playmaker has been linked with a move to Stamford Bridge for much of the summer so far, despite his manager's reluctance to sell.
Speaking ahead of Tottenham's trip to Old Trafford on Monday night, the United boss said that he understands the difficulties facing Redknapp with the current uncertainties at White Hart Lane.
"Tottenham have said they're never going to sell him," Ferguson told reporters. "That's what Daniel Levy has said. But then you've a feeling if someone comes in with the right offer he will be sold, and that's difficult for Harry, to work out Daniel Levy's thinking.
"I know Harry wants to bring in a couple of players, no doubt about that, because what he has achieved the last couple of seasons has been brilliant. It has been difficult for Harry getting through the summer not quite sure what's going to happen."
Modric, who joined Spurs from Croation side Dinamo Zagreb in 2008, has scored nine goals in 91 appearances for the club.