The fourth-placed Saints drew a firm line under a slip which saw them lose four straight Premier League games in a fortnight, by taking 10 points from their last 12 -- including a thumping 2-0 victory over Arsenal last time out.
"Victory against Arsenal was important," Saints boss Ronald Koeman told reporters at the club's Staplewood training centre on Friday.
"Important too was the one point against Chelsea, because we didn't have that result before," he said of the 1-1 stalemate against the leaders.
"And now we play another big one. Not long after we played them, and we lost that time because of our mistakes."
Southampton were beaten 2-1 at home by United early last month, in a game riddled with mistakes by the hosts.
"We didn't make those kind of mistakes against Arsenal, and so then (we know) we can beat them."
Electric winger Elia joined Southampton on loan from Werder Bremen at the start of the month, and Koeman said he would be in the frame for featuring at Old Trafford on Sunday.
"He's looking good," Koeman said. "He has a lot of ambition to show his qualities.
"Tomorrow (Saturday) we will decide if he starts or is on the bench."
Elia could slot in as a direct replacement for the injured Sadio Mane, who has been called up by Senegal for the African Cup of Nations.
Saints lie fourth on 36 points from their 20 matches so far, one point behind their opponents, and 10 behind Chelsea and Manchester City.
(Editing by Justin Palmer)
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