Scans confirmed the skipper's worst fears, identifying ligament damage that requires surgery and could put him out until mid-December with the Old Firm derby looming shortly after Christmas.
The 26-year-old sustained the injury against Rangers at Ibrox last month and he will also miss Scotland's critical Euro 2012 qualifiers against Liechtenstein and Spain over the next week.
Brown told the Daily Record: "I have been told I'm looking at up to 10 weeks out and I'm absolutely sick.
"I'd been hoping for good news but I could tell when the doctor checked me that something wasn't right. I had a bad feeling in the pit of my stomach and my worse (sic) fears have been confirmed.
"It isn't the same problem I had with my ankle - it's a different ligament altogether, although it may have been caused by the stress the old injury was causing.
"I am devastated. I feel I can't catch a break at the moment after all the injury problems I had last season.
"The last thing I needed was another lay-off like this. All I want to do is be out there helping my club and country."