Liverpool are battling for fourth place this season while United are hunting down a record 19th title.
Liverpool finished just four points behind United, who won the title, last season.
“I thought it was an exceptional season for them,” said Fergie.
“I thought it would be difficult to achieve that again. That was just my opinion.”
When asked if Liverpool had over-achieved last season, Fergie said: “Yes. I think they did very well.
“When they got that run going they showed form and consistency – that helps.
“Their challenge last year was obviously championship-winning form, but sometimes you peak with a particular team and it is sometimes difficult to do again.
“I think everyone expected Liverpool to be better, but they haven’t been, and the challenge for them, along with a few others, is to get fourth spot.
“We can all say with some certainty that one of ourselves, Arsenal or Chelsea are going to win the league.
“But no one has any certainty about fourth place, it changes almost every week.”