Defeat for Manchester United to Crystal Palace will put three year plan in question
After losing 1-0 to Manchester City, United have taken just 13 points from their opening 10 games, their worst start to a league campaign since 1986.
"It's not good enough," Van Gaal, 63, said. "But on the other hand, we are in a process, as I have said from the beginning.
"The process shall take more than one year; it shall take three years."
"I feel for the fans in the first place and the board because they have a great belief in me, my staff and my players," he added.
Defeat to Crystal Palace on saturday will make three years seem a long way away. United are only 4 points better off than the London club who are one place off the drop zone. United are nearer the bottom club than the top.
United broke the British transfer record when they signed Angel Di Maria for £59.7m in the summer, but they have won only three Premier League games so far.
They lost to newly promoted Leicester in September and drew 0-0 with bottom of the table Burnley.
They also suffered a shock 4-0 defeat by MK Dons in the Capital One Cup in August.
But Van Gaal's side have struggled with injuries, with defender Marcus Rojo now ruled out for about six weeks after dislocating his shoulder on Sunday.
"We don't need to operate on him, which is a relief," the former Netherlands coach said. "Always with these injuries, I cannot change that."