Jose Mourinho's side head to Portugal top of Group A after winning both of their opening games, while Benfica are bottom of the pile and yet to score a point.
The Portuguese champions were beaten 2-1 at home by CKSA Moscow in the opening round of fixtures, before being thrashed 5-0 by Basel last time out, but Young feels they are still quality opponents.
"Benfica haven't won the Portugese league for the last four seasons for no reason and they haven't got to the last eight and the 16 for no reason either," he told a pre-match news conference.
"We are expecting a tough game. Obviously they haven't got off to such a good start in the Champions League but when Man United turn up it's going to be a different story and we feel it's going to be a tough game."
Young has started both of United’s Champions League games this season and spoke about how he is willing to play at full-back, having spent most of his career as a winger.
"I think it’s one of those things as a professional player now that you have to be able to play in different positions and be versatile," he added.
"I’ve found myself playing at full-back, enjoying it and playing well. Wherever the manager wants me to play, I go out and give 100 per cent. Whenever I pull on the United shirt I look to do as well as I can."
Young is enjoying a run in the first team this season, having started all of United's last four games, including their 4-1 win over CSKA Moscow last time out.