Moffat admitted Smith's decision to leave was a "tough time", and he was disappointed that he "couldn't get him to stay" on the BBC One show.
However, the showrunner hasn't given up hope that Smith will don that bow tie again, just as he did for his surprise post-regeneration cameo appearance in 2014, reported Digital Spy.
"Matt is quite open about how much he misses it, and how much he wishes he hadn't left," Moffat said.