It was indefensible from Stephens, who had earlier let his team down with some shoddy defending for Arsenal's first two goals, courtesy of Pierre-Emerick Aubameyang scoring his sixth goal since his January arrival from Borussia Dortmund and a deflected strike from Welbeck.
However, substitute Charlie Austin pulled the Saints level with a tap-in in the 73rd minute.
Welbeck missed a great chance to put Arsenal back ahead when he fired over from three yards out after Wilshere had hooked the ball across goal.
The match ended on a chaotic note as Stephens saw red for lashing out at Jack Wilshere, who himself earned a yellow card for his part in an injury-time scuffle, while Mohamed Elneny was also sent off for a shove on Soares.
Southampton's defeat kept them in the bottom three, three points behind Crystal Palace but with a game in hand. Irish worldwide Long's goal was only his second in nearly 50 games for club and country but even before that the visitors made all the chances. They hold a 4-1 advantage from the first leg of the quarter-final.
Gold, gold, gold for record-breaking Aussie cyclists
Horton became Australia's first 400 metres freestyle victor at the Commonwealth Games since the great Ian Thorpe with his victory.
South Korea's ex-president Park Geun-hye found guilty
Blue House said in a statement after the verdict: "Each person must have different feelings about former President Park Geun-Hye". Park was also indicted for accepting 59.2 billion won in bribes from Samsung, Lotte and SK in exchange for business favors.
Expert doesn't expect new Rohingya boat people exodus
Malaysia's standard policy has been to turn away refugee boats attempting to make landfall, unless weather conditions are bad. In the video, Abu tells the interviewer; "We are moving because during the monsoons, the water rises very high here".
Arsene Wenger admits Arsenal made life hard for themselves in the 3-2 victory over struggling Southampton on Sunday. "Clearly we were quite prepared to allow Arsenal a lot of possession in front of us, but our intention was to counter quickly and put Shane Long in areas high up the field where we could get support to him".
Arsenal started the match with four wins on the trot fully expecting to make it five on a wet afternoon in front of swaths of empty seats and a awful atmosphere.
The 18-year-old has been described as one of the brightest young talents to have come up through Arsenal's academy, and today he made his full Premier League debut against Southampton. It was the England forward's first league goal since September. Long turned in Højbjerg's wayward shot, only for the flag to go up for offside.
"It's a competitive game, Saints are in a hard position, same as us", he said.
"I'm happy with the assist, glad to get the goals".
We have gone through a hard spell, but because the players are proud we came back.