The 30-year-old was in the stands at Croke Park for last night's dramatic 2-2 draw with Italy in which the Republic clinched a play-off spot as the holders ensured they will finish top of Group 8.
Duff spent the latter part of last week back in London having treatment on the calf problem that ruled him out of the game, but returned to training in Malahide this morning.
Manager Giovanni Trapattoni said: "Damien Duff is with us and he said to me the injury has gone."
Midfielder Glenn Whelan is suspended for the Montenegro game after picking up his second booking of the campaign last night. Trapattoni hinted he will make changes with five men - Shay Given, Aiden McGeady, Keith Andrews, Paul McShane and Leon Best - all on one yellow card and knowing another would rule them out of the first leg of the play-offs. Given, along with Kevin Kilbane, is in line to win his 100th cap if selected.
The draw for the play-offs will take place next Monday with France, Portugal, Russia and Greece looking as if they may be the four seeded teams against which the Republic could be drawn.