O'Brien struggled with the same problem last year, but Schmidt, whose side face a trip to Scarlets in the opening round of the RaboDirect PRO12 on Saturday, insists it's not a season-ending set-back.
He told the Irish Independent: "Sean's most recent operation was last week. He's feeling pretty good about the operation.
"It will be a long, slow process. There weren't any complications. It was two different operations. We're hoping he'll be back mid-December."
Other long-term concerns for the reigning Heineken Cup champions are Eoin O'Malley, Rhys Ruddock and Dave Kearney while Luke Fitzgerald won't be back for the Irish province until 2013.
"A couple may be back next week," added Schmidt. "The vast majority will come back in weeks three and four. One of the difficulties for us is suddenly implanting ten changes in your starting line-up.
"The problem is you would have no continuity with them. So they're starting back on the field with us and we'll integrate them as best we can."
63x46.png){kind=logo}