The 25-year-old was stretchered off in the first-half of the 3-1 win at Dundee and is unlikely to play any further part this season.
"It's a major disappointment for Andy personally, and for the club as well," Brown told BBC Scotland.
"He's a most unlucky guy - it was a pure accident. Very quickly it was apparent that it was a major injury."
"He clashed with Declan Gallagher of Dundee, and he fell to the ground and you could tell there was something seriously wrong with him because Andy's a very tough guy and would never lie down unnecessarily.
"With the medical provision we have nowadays I imagine it won't have serious consequences to his career - but certainly to this season's play and to our own club it's a major disappointment."