Ferguson warned his squad that the Portuguese side would not be an easy opponent in Lisbon on Wednesday, according to reports from United's official website.
"The opening game is the toughest for us. Benfica are always a handful and we always expect a difficult game. I watched a couple of their games away from home last season and they were absolutely brilliant," the Scot warned.
"I think the hardest part for us will be picking the midfield in that particular game. That area will be the biggest challenge.
"They operate with three midfield players in there. They don't play anyone off the front. They play three actual midfield players so we'll have to have a look at that."
The Scot hinted that he could put out a far different side against Benfica compared to the youthful sides he has played in the Barclays Premier League so far.
"My team in Lisbon might be different on an issue of experience," said the wily tactician.
"We've also got Darren Fletcher back fit. He's looking good and did well with Scotland last week. I think we'll need to have more experience in the game.
"The important thing is to qualify. Benfica away from home is the kind of the game you have to respect", Ferguson concluded.