Juan Mata's deflected strike three minutes from time gave the Blues a 1-0 victory over a below-par United at Old Trafford and restored their advantage over Tottenham and Arsenal in the Barclays Premier League.
Benitez's men, who have a healthy advantage on goal difference, can now effectively secure a top-four spot with victory over Spurs at Stamford Bridge in midweek, and the Blues interim manager said: "It is massive for us. We knew that we have to get six or seven points to guarantee the top four and we have got three points here."
He added: "Now we play against Tottenham and we are in the driving seat."
Chelsea had the upper hand throughout against a United side who appeared to have let their intensity levels drop after securing their 20th domestic title.
They still did enough to frustrate the Londoners but even though a draw would have been a good result for the visitors, Benitez claimed he did not want to settle for 0-0.
He said: "When the goal came we were really pleased and convinced it was an important step forward. Obviously United, after winning the title, normally you can expect they are a little bit more relaxed.
"My team talk at half-time was trying to increase the tempo because I could see we were controlling, doing well but were still missing the high tempo that is necessary if you want to beat a team like United.
"I think we did it in the second half. It still wasn't perfect but enough for winning the game."
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