Ferguson backed McDermott's performance at Reading under tricky circumstances and said that if the club was considering sacking him then it should have been done in December or January.
"I never understand football," Sky Sports News quotes Ferguson as saying. "If they had done it in December or January maybe I would have understood it because he could have brought players in.
"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."
Eamonn Dolan has been placed in temporary charge of the club.