Northern Ireland's hopes of qualifying for the 2013 European Under-21 Championship suffered a setback as they lost 1-0 away to Serbia on Friday.
Luka Milivojevic scored the winning goal in the 17th minute and Steve Beaglehole's side then had Conor McLaughlin sent-off after 50 minutes for deliberate handball.
Steve Beaglehole's men remain top of Group Four and are in action again on Tuesday when they face Denmark at The Oval in Belfast.
A win will be vital at the home of Glentoran, with Northern Ireland having played at least one more game than all of their rivals, with Denmark and Macedonia still to kick-off their campaigns.
Northern Ireland had beaten the Faroe Islands 4-0 at the Oval in August.
They drew 0-0 in their Group Four opener against the same opposition.