The Red Devils have endured a raft of injuries at the back over the course of the season, with Antonio Valencia, Luke Shaw, Marcos Rojo, and Ashley Young all currently out.
"When you see our back luck with full-backs - Valencia, Shaw, Rojo, Darmian, Young - Darmian is now fit but the others are all injured. I need full-backs," Van Gaal told a press conference on Friday.
"[Cameron] Borthwick-Jackson and [Guillermo] Varela are doing great but the level at United is high. It is very difficult for youngsters to do that every week.
"That is why we have to look for reinforcements. We are looking but the clubs don't want to sell. Everybody thinks it is easy but it is not so."
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"I don't think that the press have to help me with my decisions," the Dutchman said when asked about his job security. "First they have sacked me, now they are interfering with my future. I don't have to answer.
"I have a relationship with the players of Manchester United, that is the most important thing. Not everything the media writes is true."