Cullen played the whole game for Ireland but the West Ham United player couldn't prevent Serbia securing a 3-1 victory.
The game was the last in Ireland's UEFA European U21s Championship qualifying group.
Ireland took the lead through Everton striker Courtney Duffus but three unanswered Serbian goals in reply helped to clinch the win for the visitors.
After picking up his third U21s cap of the season, Cullen is now expected to return to Apperley Bridge at the start of next week to resume training with his City team-mates.