Ferguson's side face a new-look Liverpool at Old Trafford today bidding to register a convincing win after a number of below-par displays at the start of the season.
Victory against their biggest rivals, now under the guidance of Roy Hodgson, would be a timely lift for United, and Ferguson turned up the heat ahead of the contest by insisting Benitez was only given a stay of execution at Anfield due to his contractual situation.
The Spaniard, who penned a new five-year deal at Anfield in 2009, eventually left the club in June having guided the reds to a disappointing seventh-placed finish in the top flight.
"I wasn't surprised at all that Liverpool changed their manager in the summer. I think everyone expected it even though Benitez had signed a new contract," Ferguson said in quotes reported by The People.
"That was the only complication about it. If he hadn't signed a contract the previous season then I am sure he would have gone before the end of the last season.
"But the fact he had signed a new contract brings its own complications in terms of compensation and that type of thing, so they waited.
"Obviously, the directors at the club had come to the stage where they had to discuss the manager's future because of the performances. Then I think everybody expected it to happen and for him to go."
Meanwhile, Ferguson has criticised his players for lacking concentration after dropping costly points in recent league matches against Fulham and Everton.
"To lose two points in added time once is perhaps bad luck, but to do it twice this early in the season is just downright careless and it has to stop," Ferguson said.
"I think back to that great quartet of strikers - Dwight Yorke, Andy Cole, Teddy Sheringham and Ole Gunnar Solskjaer - who all had a ruthlessness and concentration that saw them determined to finish off games.
"It's a quality our present team must acquire and I want the players, both up front and at the back, to reflect on their powers of concentration - or lack of - and work on that aspect of their game."