The Life On Mars star appeared as The Doctor's arch-nemesis in five episodes between 2007 and 2010.
At a recent Doctor Who screening in New York, Moffat suggested that Simm could reappear in a future episode.
"After [his last appearance] he'd been saying, 'I think now that David [Tennant]'s left as the Doctor, I would have to leave the Master'.
"He [later] pulled me aside and said, 'I didn't mean that! Look at me, I'm fit, I'm okay!'"
Moffat also dismissed the suggestion that Sherlock star Benedict Cumberbatch could replace Simm.
"Benedict has to wait in line probably," he said.
The new series of Doctor Who premieres on April 23 on BBC One in the UK and BBC America in the US.
> John Simm: 'My Master is unhinged'