Speaking at a press screening in London of the Christmas special of the show, called The Next Doctor, he said replacing the Doctor could take a while.
He said: “It’s a big deal to set up. Whoever becomes the Doctor has got to take on a whole life. It’s a lot of soul-searching to do.”
The special, which premieres on BBC1 on Christmas Day, sees current Doctor David Tennant star without an assistant, alongside David Morrissey, who appears as another Doctor.
There has been speculation that Morrissey could be poised to take up the starring role, as well as Martin Clunes, David Walliams or a female Doctor.
Russell was asked at the screening whether he would like to see a woman or a black Doctor take over when Tennant bows out at the end of next year.
He said: “I think the more it’s talked about, the more likely it is to happen.”
When asked about women he would like to see in the role replied: “Lesley Sharp or Catherine Zeta Jones.”
Tennant is recovering from surgery for a bad back, which has forced him to pull out of his role in the Royal Shakespeare Company’s Hamlet until he gets better.
Russell said: “We’ll have to be very careful with him – I don’t think he will be swinging on a wire on the first day.”