Both Bailey Peacock-Farrell and Daniel Ballard have returned to their respective clubs from the international camp due to injury issues.
But the manager remains encouraged by overall progress as his players work from Stockport County’s Carrington training ground over the next few days before flying out to Hungary on Thursday for Saturday’s behind-closed-doors date with Belarus.
The international boss is determined to maximise the squad’s extended preparation time in England ahead of facing Belarus and Bulgaria.
“This is a great base for us,” said O’Neill. “We are developing as a team and there are good facilities here to work in.
“We just want to continue to progress.
“We have a longer lead-in time to this game (against Belarus) than for previous games…so we’ve got to make the most of the time we have here.”
Peacock-Farrell’s squad exit due to a shoulder injury will prevent the Birmingham City goalkeeper from securing a 50th cap across the international window against Belarus and Bulgaria at home next Tuesday.
Ballard has returned to Sunderland for treatment on his ankle injury.
O’Neill is expected to operate with a 23-strong panel, with the Irish FA stating “there are no plans at this stage to call up replacements for the pair”.
Northern Ireland sit third in the current League C Group 3 standings after kicking off the fourth Nations League campaign by beating Luxembourg in Belfast then losing out away to Bulgaria.
Former Irish League striker Ronan Hale is still waiting on international clearance but could be available to Northern Ireland for next month’s fixture window at home to Belarus and away in Luxembourg.
The Ross County summer signing – and former Republic of Ireland youth international – now sits on seven goals in 11 appearances for the Scottish Premiership side, including a penalty on Sunday against Celtic.