United struggled to break down the resilient Saints and after a goalless first half, substitute Charlie Austin came off the bench to head home the winner late on to condemn the home side to their sixth defeat of the season.
The result heaped yet more pressure on Van Gaal, whose side have only won two of their last 10 Premier League games to leave them fifth in the table and 10 points behind leaders Leicester City.
And Van Gaal conceded that he could see why United’s fans were so disappointed with the manner of Saturday’s defeat at Old Trafford.
Speaking to Sky Sports after the defeat, Van Gaal said: “They are right to boo. I cannot deny that, I saw the match. For better or for worse we have to stick together.
“The two teams were well organised but didn’t have the creativity to create chances. The lucky one has won but the fans want to see a winning Manchester United and we could not provide that for them.”
United are now preparing for their FA Cup fourth-round trip to Derby County on Friday night.
MORE: The latest football news