Manchester United manager Louis van Gaal says it could take three years for the club to reach their full potential.
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.
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 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."