Louis van Gaal called on Manchester United fans to rally round their team despite yet another disappointing defeat.
Southampton manager Ronald Koeman piled pressure on his old foe by beating United 1-0 on Saturday thanks to Charlie Austin's 87th-minute winner.
A poisonous atmosphere took hold at Old Trafford after the final whistle, with supporters from the Stretford End and South Stand heckling Van Gaal as he walked along the touchline before he disappeared down the tunnel.
Boos also echoed around the stadium at half-time after United failed to score in the first period of a home match for the 11th consecutive game.
The message was clear - Van Gaal is no longer wanted by some United fans and they are increasingly unhappy with the slow, dour football that is often on display at Old Trafford.
The United manager understands the fans' angst, but claims now is the time for unity, rather than mutiny.
"They are right (to boo). I cannot deny that," Van Gaal said. "I have seen the match also and I cannot deny it but also in better or for worse, you have to stick together."
Daley Blind registered United's first and only shot on target in the 13th minute and thereafter the hosts put on a dull display, regularly shuffling the ball sideways and backwards rather than forward.
United fans had seen enough by the time two desperate long punts up field went awry and they responded with loud boos and chants of "attack, attack, attack".
Van Gaal suggested his players were struggling to deal with the demands of playing for such a high-profile club.
He added: "You have to meet expectation and that expectation is very high, and that gives a lot of pressure for the players.
"We are working very hard but also we have a lot of injuries and that I cannot change with words.
"It's also a lot of bad luck when you have five full-backs injured so you cannot prevent that."
The latest full-back to fall victim to United's injury curse was Matteo Darmian, who went off in the second half while spitting blood following an aerial collision with Shane Long.
Van Gaal felt the loss of Darmian, and the withdrawal of the ineffective Marouane Fellaini at half-time, left his side vulnerable to aerial assaults and he was proven right when debutant Austin headed in James Ward-Prowse's free-kick seven minutes after coming off the bench.
"I had the fear because they could only score from set plays as they didn't create other chances and our defence in set plays was weaker because I changed Fellaini and had to change Darmian so every change meant we were weaker in set plays," he said.
"That was my fear but I wanted to win this game also that is why I changed Cameron Borthwick-Jackson for Adnan Januzaj. He could have scored but that is the risk you take."
United are now five points behind Tottenham in the hunt for Champions League qualification with 15 matches to go.
Saints boss Ronald Koeman was delighted, not only to see debutant Austin score but also his link-up play with Shane Long, who played the full 90 minutes in the win.
"We know that we like to sign Charlie Austin because he creates numbers of goals and he showed it with Shane (Long) together in the last ten or 15 minutes, they did a great job," Koeman said.
"It's not fair to mention one player because the whole team did a great job."
"It's sometimes strange to explain why you lose four or five in a row and why now you win three in a row, but that's football," the Saints boss said.
"It's difficult to understand sometimes, but we need to keep the spirit and our level. Now we have big competition in our squad and I have to make difficult decisions."