The 52-year-old has called on his players to work as a unit in order to progress to the semifinals but is well aware that they go into the game at Old Trafford as the underdogs.
United is 12 points clear of Manchester City at the top of the Premier League and Benitez believes a lot of its success is down to depth in offensive areas.
"It will be a tough game against a good team who are top of the table," he told the club's official website.
"It's an away game for us, so we know it will be difficult but we are confident we can go through.
"If you analyze the squad they have, they have four strikers and two players in each position that could play for any team. They have a strong squad and even when they make changes they keep the quality.
Ramires, Victor Moses and Ashley Cole were all rested for Chelsea's game against Steaua midweek and Benitez is hoping their reintroduction will reinvigorate the side after a 1-0 loss in Romania.
"It's an important competition which Chelsea have done very well in, so hopefully we can carry on," said the Spaniard.
"The good thing is that we have another game so soon after a defeat, hopefully we can do better and win, and we will have five or six players with fresh legs."