Van Gaal, whose side face Crystal Palace on Saturday in the FA Cup final, said: “Of course [I understand the boos], because I have also said that the expectation is very high, especially the fans.
“They are expecting a lot. But I think these expectations are much too high, and we are a team in transition, and that I have said when I started here.
“Maybe I have to bring over the message much clearer than I have done.”
United have been criticised heavily for their performances this season, having dropped out of the Champions League group stages in December and the Europa League to Liverpool at the round of 16 stage.
Van Gaal says that while a lack of goals has been a problem, the big issue was the injuries his side had to deal with in defence during the winter.