The Scotland captain will miss Thursday's crucial Europa League clash with Ajax at Parkhead after damaging a ligament in Saturday's goalless draw against Kilmarnock.
That match marked Brown's return after four games out with a different knee complaint and now a scan has revealed the extent of his latest injury
Three months is the prognosis but the 30-year-old Hoops midfielder remains positive about his ability to deal with the setback
"I was a bit devastated," Brown told Celtic's official website
"When I woke up in the morning, I had a wee feeling it was my ligament.
"I had hoped it was a wee strain but it was a bit more serious than that.
"I will be on crutches and my leg will be in a brace for four weeks and then after that I will see how it's healing.
"If it's healing well it will be good news and I won't need an operation, but if it's not going so well I might have to go down the operation route.
"It's always hard being injured
Coming back from an injury, playing 90 minutes and then being out for possibly three or four months is difficult.
"But I've had these blows before in my career and I will get over it - I will get through it no problem.
"Part and parcel of playing football is that you are always going to have injuries.
"It's about how you come through them and how you look after your body over the next three or four months."
Source : PA
Source: PA