The 27-year-old actress, who played assistant Rose Tyler to David Tennant's Doctor, claimed that Matt Smith has "big shoes to fill".
"I hate it that he's left!" she admitted. "But then I know Matt Smith really well and I love him and I think he's such a brilliant actor.
"The role couldn't have gone to a better guy. He's capable of that. He's got big shoes to fill, but I'm certain he can do it."
Piper also revealed that she is yet to watch Tennant's final scenes, but said that she will be upset when she does.
"I haven't seen it yet! I went away for Christmas and New Year and it's a bit weird trying to sly off and watch an episode of Doctor Who on my own! I definitely will, though," she said.
Piper will next be seen in series three of ITV2's Secret Diary of a Call Girl, which begins Thursday, January 28.
> Click here to read our full interview with Billie