The manager and his Saints team host United in the Premier League having scored in only two of their five league fixtures played so far this season.
United, however, have also looked unsettled and vulnerable
They lost 2-1 at PSV Eindhoven on Tuesday and by the same scoreline at Swansea before the international break while Daley Blind, playing in the unfamiliar position of central defence, has been criticised by former United defender Rio Ferdinand as their "weak point".
Despite Chris Smalling's improvement, United's defence has, by their standards, been weak since Sir Alex Ferguson's retirement and Ferdinand's decline
That Luke Shaw broke his leg against PSV has further weakened Louis van Gaal's options and Koeman believes that his team are capable of capitalising on the uncertainty among their opponents' back four.
"It's a good game to show that we can play attacking football and create chances," Koeman told Southampton's official club website
"That's important for Sunday to win the game.
"We have to be confident
If we are not you can't win
You have to show your qualities and your belief in a good result on Sunday.
"They are a big team, a big club
A lot of attention comes to everybody
That makes the game more interesting
I think that's the best word for Sunday.
"Man United is always a team who tries to play football, tries to play football from the back
Normally they have good ball possession.
"It of course depends on Wayne Rooney or if Marouane Fellaini will play as the striker, but the intention from the Man United team is always to control the game and to dominate.
"But we have our own style of play
It's different because they will play different if it is Rooney or Fellaini in front, but that doesn't mean any difference for our team."
Koeman, incidentally, is expected to recall Sadio Mane after the winger missed last week's goalless draw with West Brom.
Mane was a reported summer transfer target for United manager Van Gaal, and Koeman remains without the injured Fraser Forster, Ryan Bertrand, Florin Gardos and Jordy Clasie.
Source : PA