Southampton manager Ronald Koeman said targeting Manchester United's defensive flaws was behind the 1-0 win that put his side third in the Premier League.
The Reds failed to register a single shot on target as Dusan Tadic's goal secured the Saints' first league win at Old Trafford in 27 years.
"You analyse your opponents," said Saints boss Koeman.
"You know they have difficulties building up with three centre-backs. We have had good organisation all season."
Koeman added: "We now have 11 clean sheets in 21 games and that is key to being successful."
Southampton leapfrogged United in the table following their victory and are three points clear of fifth-placed Arsenal as they look to secure the top-four place which would give them Champions League football next season.
And Koeman, who is in his first season at St Mary's after replacing Mauricio Pochettino in the summer, says European ambitions are now realistic.
"We have aspirations [of securing a European finish], but at the beginning of the season you have to wait," said Koeman. "Now we can say we fight for a European place.
"The top five or six, we will fight until the last game."
However, the manager did say he will be talking to goalscorer Tadic after the substitute was booked for celebrating his goal by taking off his shirt.
"I don't like players taking off their shirt," said Koeman.
"It is a yellow card and a stupid fault. You get another and it is a red.
"When it is the last minute of the season and you have qualified for Europe, it is no problem. I don't like that. Keep your shirt on."
He did, though, have praise for the match-winner: "He was fantastic in the final half hour. He made the difference and I am proud of him."