The Red Devils are 16 points behind leaders Leicester City and the FA Cup appears to be their only hope of silverware this season despite heavy off-season investment, but they face a daunting quarter-final replay at West Ham on April 13.
Ferguson pointed out the host of injuries Van Gaal has had to deal with throughout the season, while also trying to integrate their summer additions into the squad.
"It is dead easy to be critical. You have to be realistic about some things. The number of injuries he has had... if you have players missing of that calibre it has to have an impact on the team," the club's most successful manager told British media.
"The other aspect which is always easy to forget, there are five new players in their first season at United and two are young players in (Anthony) Martial and Memphis (Depay), and their promise is good.
"You have to have some patience being a Manchester United fan and they have shown that over the years; in Matt Busby's time, in my time."
Van Gaal has brought through a number of academy players this term, such as striker Marcus Rashford, who has netted five times in eight appearances, and Ferguson has tipped the 18-year-old for greater things.
"He's given nine young players their debut -- I think the future is good. Rashford is an example of what I think is Manchester United... Rashford is a sensation and one of the best in years," Ferguson said.
Van Gaal's side host 12th-placed Everton at Old Trafford in the league on Sunday.
(Reporting by Shravanth Vijayakumar in Bengaluru; Editing by John O'Brien)
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