Injury setback suffered in pre-season friendly means defensive problems loom large for the Boys in Green with Dunne already absent
St Ledger will be out of action for six to eight weeks after damaging medial knee ligaments on Thursday and will miss the vital World Cup 2014 qualification match against Kazakhstan on September 7.
A statement on the club's website reads: "The Republic of Ireland international suffered a medial ligament strain in the early stages of the game at the Pirelli Stadium and is expected to be sidelined for a period of six to eight weeks.’’
Giovanni Trapattoni's side face Serbia in Belgrade in a friendly match on August 15 with the game against Kazakhstan coming three weeks later and Ireland are facing a defensive crisis.
Centre back Richard Dunne has also been ruled out for two months and is currently considering his international future, meaning Trapattoni will be forced to utilise a new partnership at the heart of his defence.
O'Brien's call up represents his first involvement with the national team since 2008.