The clip was filmed yesterday at San Diego Comic-Con and David said "Well, I know Jodie very well, I've known her for years and I can't think of a finer choice. I'm very excited!"
The interviewer then asked David why he said that and David replied "Because she's a great actress and I think that's what that part needs. I think the better the actor, the better the show and Jodie's one of the best actors working in Britain right now so I think the show has lucked out."
David Tennant started working with Jodie Whittaker on Broadchurch in August 2012 and they both appeared in all three series of the drama.
Apologies if the video does not play in your country.
DoctorWho section of DavidTennantOnTwitter.com