O'Brien, who recently won the Heineken Cup with Leinster, played for Ireland 'A' in their opening match of the 2009 Churchill Cup - Wednesday's 30-19 defeat of Canada in stormy conditions in Glendale, Colorado.
The Carlow man sustained a knee injury during the second half and has been ruled out of the next two games in the tournament, so he will return home to Ireland to begin his recovery.
Galwegians clubman Browne, O'Brien's replacement, is a versatile forward who can play in the second row or at blindside flanker.
It will be great experience for the 22-year-old, working with his Declan Kidney and his senior coaches Stateside, and an Ireland 'A' debut could well be in the offing.
Browne joins five of his Connacht team-mates in the Churchill Cup squad and encouragingly, he is the fifth player from that Under-20 Grand Slam-winning side to make the Kidney-coached panel for the tournament.
O'Brien, Cian Healy, Felix Jones and Niall O'Connor are the others.