Louis van Gaal admitted Manchester United's supporters were right to react angrily as Southampton won at Old Trafford.
An air of restlessness throughout 87 goalless minutes turned to outright mutiny as Charlie Austin headed Saints to a second straight 1-0 win on United territory.
United are five points off the Champions League places and lost another defender to injury as Matteo Darmian - used as one of three center-halves on the day - was taken to hospital with a chest injury following a collision with Shane Long.
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Lengthy treatment for the Italian interrupted United's most promising spell of the match, and Van Gaal offered the club's lack of fit full-backs as a mitigating factor without disputing the crowd's angry reaction.
"You cannot say they are not right," he said. "They are right, and of course they are disappointed and have the right to boo me, but we have to stick together in this period because we have a lot injuries."
Asked if his relationship with the fans is at a new low he said: "Yes, because it was very clear."
Austin is a proven Premier League goalscorer but was left unmarked to score the winning goal on his debut shortly after coming off the bench.
Van Gaal said: "It was not good today. We lost and in our defense you could know that our defense at set plays became every minute weaker because I have to change players.
"That was our problem in the last minute, but it was a poor game. We could not create chances and our opponent had also few chances, so it's more or less a 0-0 game, but at the end we have lost.
"Our opponents play with five defenders and are very compact but still we have to create chances."
Asked whether Darmian's injury was a turning point he said: "Yes. In the second half we started very well and the crowd was behind the players, but then the injury to Darmian was not very nice. I think that plays a role but that is no excuse."
Van Gaal revealed Darmian had been "spitting blood" before he was taken to hospital, saying: "It was not nice for the players to see that."