The Football Association of Northern Ireland has merged the traditional September and October breaks into one continuous 16-day window, resulting in four international fixtures instead of two.
Northern Ireland began this campaign with a strong performance, securing four points from their opening two matches.
However, the squad faced significant challenges before departing for Slovakia to face Ukraine later in the extended window.
Key players including Conor Bradley and Ali McCann were already unavailable prior to the trip.
Sunderland defender Daniel Ballard suffered a hamstring injury in Georgia, further ruling him out for the remainder of the period.
In previous seasons, the Nations League campaign would have concluded with the match against Hungary, allowing players to return to club duties immediately.
The squad previously undertook a 5,000-mile round trip to Georgia where Shea Charles scored a 99th-minute winner to secure a late victory.
Following that game, the team flew back overnight to arrive in Belfast on Saturday morning for their first home match in 315 days.
Unlike German manager Jurgen Klopp who can utilise two distinct squads, O'Neill must manage a smaller playing pool with limited resources compared to Europe's top clubs.