Cormac Devlin issued the warning, privately shared by a number of Fianna Fáil TDs, as Ms Hanafin refused to rule out a potential future leadership challenge should she make a remarkable return to the Dáil.
On Monday evening, Mr Devlin won the hotly contested selection convention by 68 votes to 64 on the second count, after transfers from defeated candidate councillor Kate Feeney took him over the line.
However, as was widely expected, the party’s national constituencies committee yesterday added Ms Hanafin to the ticket, due in part to the gender quota and that she is believed to have the better chance of winning a seat.
Mr Devlin said he has a “strong working relationship” with his colleague but a one-candidate strategy is preferable.
“Obviously this decision dramatically reduces the chances of Fianna Fáil securing a seat,” he said.
Ms Hanafin told RTÉ radio she has a “big challenge” ahead of her but took solace from Fine Gael Justice Minister “Frances Fitzgerald who actually lost her Dáil seat, came back, and was made a minister, and now you’re talking about her as a future taoiseach”.
Fianna Fáil is widely predicted to win only one seat in the Dún Laoghaire constituency, with sitting TDs privately concerned Ms Hanafin’s name on the ticket could split its existing vote.
Meanwhile, Government ministers say they do not believe a November election is on the cards despite growing speculation of a post-budget snap call.
Health Minister Leo Varadkar and Simon Harris, the junior finance minister, both insisted the vote will occur next year. Mr Varadkar said: “I haven’t ordered the posters yet, but I will do, just in case.”