Ex-Arsenal defender Martin Keown says Louis van Gaal's 3-5-2 Manchester United system needs changing, because "it doesn't work".
Keown told BBC Radio 5 live that their players are struggling with the formation.
"He's persisting with this three at the back, and I don't believe the players are picking it up well enough. They don't seem to understand it," he said.
Keown's assessment came after United's 4-0 League Cup defeat by MK Dons.
Their thrashing at Stadium MK was the second time the former Premier League champions had lost in the second round of the competition. The last time was in the 1995-96 season, where they suffered a 3-0 defeat at home to York City to go out 4-3 on aggregate.
Former Chelsea, Everton and Scotland winger, and BBC Sport pundit Pat Nevin believes Van Gaal needs "two or three world class players who are adaptable" in order to make his preferred system work.
Nevin said: "Phil Jones on the right hand side may well benefit from Louis van Gaal's love of adaptability.
"But the most important player is arguably the skipper. Wayne Rooney, I would suggest, is the most versatile Englishman playing football today.
"If he can get the best out of him in a variety of positions, in a variety of systems, then that could be the key to success for Manchester United and Louis van Gaal."
Ian Stirling, vice-chair of the Manchester United Supporters' Trust, said the fans would be patient, but that the consensus was the club were starting to pay the price for under-investment in the team in recent years.
He said: "The real fear is that financially we are playing catch-up. We lost a lot of ground last season and Van Gaal is traditionally a slow starter as a manager," he added.
"We have an understanding of his philosophy and what he's achieved before.
"He didn't take over a fantastic squad. We lost a lost a lot of experience including Ryan Giggs and then in defence with Ferdinand, Evra and Vidic."
The signing of Angel Di Maria takes United's summer spending to £132m, with Marcos Rojo, Ander Herrera and Luke Shaw brought in.
Former Manchester United midfielder Paul Scholes, writing in his Independent column, said the three at the back system "doesn't seem to be working" and that United "look like they are going to concede goals too often".
The 39-year-old, who retired in 2013, said it could take longer than three months to re-shape the side, but is hopeful Van Gaal's "capable of rebuilding".
He said: "I understand that Van Gaal is trying to make a lot of changes very quickly, but the problem is the players don't seem to have adapted comfortably to 3-5-2 yet.
"It is never easy to make a dramatic tactical change in a short space of time when you are dealing with players who have been drilled to play with a flat back four for many years."