© Getty Images
Southampton frustrated the hosts throughout at Old Trafford and Louis van Gaal's side finished the match having had just one shot on target - a speculative first-half effort from Daley Blind that was easily saved by Fraser Forster.
United created very little in the way of chances in the opening period and it was the visiting Saints who had the only opportunity of note when Sadio Mane and Dusan Tadic exchanged passes before the former just failed to bring the ball down and shoot with only David de Gea to beat.
With the match goalless at half time, United failed to score in the opening 45 at home for the 11th match in a row.
Moments later Southampton went close to the opener when a Tadic corner was nodded just wide by Victor Wanyama.
It looked as if United would be heading for a fifth goalless draw at home in the league this season until the 87th minute when Austin was left unmarked to head home a corner from another Saints substitute, James Ward-Prowse.
Three points sees Southampton end a four-game away losing streak and climb up to eighth. They are now just four behind United, who drop five points behind the top four with Tottenham Hotspur beating Crystal Palace.