The Everton striker has pulled out of Sunday's qualifying match with Sweden with a calf injury which forced him to miss his club's 1-1 Premiership draw with Coventry on Monday.
Ferguson's absence from the tie at Ibrox means the player will have missed 15 of the last 17 games his country has played. The pounds 4m forward has made only two appearances since his transfer from Rangers two years ago and has taken four years to earn just six caps - his last, against Austria in Vienna in August, was his first involvement in 20 months.
Brown said: "It's not exactly a surprise to me. Duncan has missed so many games now that I'm quite ambivalent about it. But he could have had more than 30 caps by now. If you are an international manager you have to get used being let down by players. Obviously, the striking position is one we are not well-endowed with, so it's disappointing to lose Duncan."
Scotland will learn from Fifa, the governing body of the world game, tomorrow whether they are to get the points from the controversial Estonia match or be forced to replay the fixture, and will be told if their captain Gary McAllister's one-match ban has been wiped out or not by the farcical episode in Tallin.
The Wales captain, Barry Horne, is the latest injury casualty before Saturday's World Cup qualifier against the Netherlands in Eindhoven. The 34-year-old Birmingham midfielder had to pull out because of a hamstring strain and his place has gone to Bristol Rovers' Marcus Browning.
Bobby Gould, the Wales manager, had already lost Ryan Giggs and Nathan Blake to injuries. Mark Hughes is suspended.Reuse content