Last season's Champions League winners are in an all-London fight for third and fourth place, with one of Chelsea, Arsenal or Tottenham poised to miss out on a place in the Champions League next season.
Following an unbeaten run of four wins in the last five, which has included Chelsea advancing to the Europa League final, Benitez is upbeat about securing a place in the top four of the Premier League and believes his team can win at Old Trafford.
"As soon as you can win some games in a row you have momentum and confidence and it is easier," Benitez said on chelseafc.com.
"We are not doing too many things in a different way but we are more convinced that we can beat anyone.
"We are creating chances and we are controlling the games. Overall it is a good period, the team has more confidence and that is the key."
The consequences of finishing fifth are unthinkable for the Blues and Benitez believes his side will qualify for the Champions League with wins over United and Spurs.
"Every single game will be important for us," added Benitez, whose side play Benfica in the Europa League final in Amsterdam on May 15.
"To get three points in every single game will be massive. We're so close.
"We know if we can get these three points [against United] and against Tottenham, we'll be in the top four."
Interim boss Benitez was named manager for the month for April, but he appears to be entering the final month of his tenure at Stamford Bridge, with one national newspaper reporting that Jose Mourinho will return to manage Chelsea on July 1.
The Spaniard was not universally welcomed when he was appointed in place of Champions League-winning boss Roberto Di Matteo in November, mainly due to his prior association with Liverpool, where he had a fractious rivalry with Mourinho.
The turnaround in Chelsea's fortunes can perhaps be traced back to two occasions.
The first, Benitez's February plea at Middlesbrough for the fans to support the team, rather than barrack him.
The second, also in the FA Cup, was on the field, when Chelsea responded from falling two goals down after 11 minutes at Old Trafford to draw and then went on to win the quarter-final replay.
Chelsea also eliminated United from the Capital One Cup and the clash is the fifth of the season between the pair.
Benitez and United counterpart Sir Alex Ferguson have endured an uneasy relationship over the years and the original FA Cup quarter-final tie featured an argument over the lack of a handshake from the Scot, who did offer his hand in the replay.
Although United have wrapped up the title, Benitez does not expect an easy afternoon for his side.
"It's always difficult," he said.
"It is a very good team. They don't have too much pressure, but still we will try to win in front of their fans. It will be a tough game, for sure."
Chelsea's future in the Champions League depends largely on the United clash and Wednesday's London derby with Tottenham, their 66th game of a season which will feature 69 games.
Following the Spurs clash is the trip to Aston Villa, the Europa League final with Benfica and then the season finale with Everton.
"It's important to use different players and keep them fresh," Benitez added.
"Now at this stage of the season we will be ready; hopefully we can keep them ready physically too.
"We have enough quality and confidence in the team. We have to do our job and hopefully the other teams will make some mistakes."