The 25-year-old wasn't included in Danny Wilson's squad for last night's defeat to Leciester City, after which the Barnsley boss revealed, " "He's (O'Brien) going for an operation on Wednesday with a cartilage tear. We've been dealt some rough cards but we've got to get on with it."
Things got even worse for Wilson last night as first-choice central-defensive duo Martin Cranie and Peter Ramage both limped off with hamstring injuries. "We've got to make sure we rise above the sympathy vote and go again," added Wilson.
The extent of Ramage and Cranie's injuries is yet unknown, although it is highly unlikely either defender will be fit for this weekend's clash at Watford.