The two sides meet at Old Trafford on Saturday, with Reading caretaker boss Eamonn Dolan given the unenviable task of trying to get something out of the game for the Premier League strugglers.
But Ferguson feels McDermott was unlucky to lose his job and is also confused by the timing of the decision.
"I never understand football," said Ferguson.
"If they had done it in December or January maybe I would have understood it because he could have brought players in.
"Brian McDermott has done a good job given the outlay. He spent something like £1.2m and got in £7.4m and got them promoted to the Premier League.
"He was manager of the month in January - I hope I am not manager of the month this month."