The Dutchman, who has managed Ajax, Barcelona and Bayern Munich in his career, has recorded just one victory in the Premier League out of five matches so far this season.
"At this moment this is maybe my toughest job but we have to see in a year," Van Gaal told reporters. "We cannot say it about one week with the injuries. As a manager you know already in advance that it's a big challenge.
"This club is in a transition. What you don't know in advance is what the hard moments will be. At the start we won all of our games but now we are in difficult circumstances.
"We should have won last weekend [against Leicester City]. We had to win when we were 3-1 up. If we had we would have been two points off second place, but because of the loss you are now questioning me. If we won then it would have been different. We will overcome things - the season lasts a long time."
United are currently 12th in the league table.