Ulster Unionist leader Mike Nesbitt last night met party members to quell dissent over his appointment of Army veteran Andy Allen to the Assembly.
There was discontent after Mr Nesbitt dispensed with convention and appointed Mr Allen in East Belfast to replace outgoing MLA Michael Copeland.
The Belfast Telegraph understands one party member threatened to quit over the matter.
The situation arose earlier this month after Mr Copeland announced he was leaving due to ill health. When such vacancies arise, the local UUP constituency association usually receives names from prospective candidates and holds a selection meeting, after which UUP officers decide between the top two.
However, in this case Mr Nesbitt appointed Mr Allen without consulting the local association.
A number of UUP members expressed shock at the move, saying they had been denied their say. They also pointed out that Mr Allen was associated with north rather than east Belfast.
The East Belfast UUP Association had been due to hold its selection meeting on Monday night to pick who it wanted from the three who had put their names forward. They were Belfast councillors Graham Craig and Chris McGimpsey, and Lisburn and Castlereagh councillor Michael Henderson.
The meeting, which was set before Mr Copeland's resignation, was scrapped due to bad feeling over Mr Nesbitt's move.
The above names, along with DRD minister Danny Kennedy's former special adviser Rodney McCune, had been regarded as likely frontrunners by the association to replace Mr Copeland.
There was also speculation that long-term East Belfast UUP constituency worker Maggie Hutton was in the running.
Mr Nesbitt called a special meeting of the association last night to deal with the discontent. It is understood that just 15 members turned up.
Councillors Sonia Copeland and Jim Rodgers both confirmed they did not attend. Mrs Copeland said she had a prior engagement, while Mr Rodgers simply said he did not attend. The Belfast Telegraph spoke to a number of members present at the meeting. They described it as a "waste of time" and said "the horse has bolted".
"People are annoyed they were denied the right to vote," one added. "This was very much a justification. Mr Nesbitt was told that people are threatening to resign over this. He just responded, 'Good luck to them'."
Mr Allen joined the UUP last year. He lost both legs while serving with the Army in Afghanistan, and since returning home he has become well-known for helping Army veterans.
There was controversy in June concerning another co-option within the UUP. When Tom Elliott resigned from the Assembly to focus on Westminster, the two candidates selected by the Fermanagh UUP Association, Kenny Donaldson and Rosemary Barton, were disregarded. Instead, Neil Somerville was selected.
The party later said a mathematical error had been made, and that Mr Somerville should have been one of the two candidates put forward by the association.