Butterworth captained Norwich in the early 1990s, and returned to the club to team up with his former team-mates, manager Bryan Gunn and first-team coach Ian Crook, in February.
Johnston also told fans at the city's Flamingo Hotel that securing manager Walter Smith's future, along with assistant Ally McCoist and first-team coach Kenny McDowall, for another year offered the club continuity.
Redknapp said first-team coach Joe Jordan and Tony Adams, the assistant manager, would be in charge of Portsmouth for Sunday's match against Fulham at Fratton Park.