The former Liverpool boss replaced Roberto Di Matteo who was sacked on Wednesday after eight months in charge.
Ferguson said: "On his CV he could have two world championships and nothing to do with the construction of the teams."
The Scot added: "Rafael Benitez is very lucky. Jose Mourinho won the treble at Inter; Rafa took over and they won the world title without having to do anything."
Di Matteo was sacked despite having won the Champions League and FA Cup in his short tenure.
"Everyone feels for Roberto Di Matteo, and quite rightly," Ferguson said. "It's quite amazing that you win the FA Cup and the Champions League [and lose your job]."
But first up for Benitez is a home match in the league against Manchester City.
"I hope he is lucky on Sunday too," said Ferguson.
"It happens time and time again when a new manager comes in that a team win their first few games.
"It is strange how it happens and sometimes you wonder if it encourages chairmen at football clubs to make changes because they think: 'if we get a new manager we'll win the next two or three games'."