Manchester United need to invest "a few hundred million" pounds in the squad if they are to compete for major titles again, former star Gary Neville says.
Ole Gunnar Solskjaer's team suffered a second Premier League defeat of the season when they went down 2-0 to West Ham at the London Stadium on Sunday.
Goals from Andriy Yarmolenko and Aaron Cresswell helped the hosts take the three points against a tame United side that has collected just eight points from the first six league games this season.
The Red Devils continue to struggle despite signing Harry Maguire, Aaron Wan-Bissaka and Daniel James to bolster their squad in the close-season.
But ex-United and England full-back Neville believes their latest collapse proves that the squad is still far below the level required to challenge Manchester City and Liverpool this time around, warning that it will take a couple of years to bridge the gap.
"Ole needs three or four transfer windows to get this squad up to a level where they can compete," Neville said on Sky Sports .
"And he needs a few hundred million quid - you can't back out of that. The days when you could pinch someone in the transfer window have gone.
"This is not going to be easy."
He added: "I'll say it again: nearly £900 million. You have to ask yourself how much more money Manchester United need to spend to get themselves back to where they want to be."
United were dealt a blow on the hour mark when Marcus Rashford was taken off injured and replaced in the centre-forward role by winger Jesse Lingard.
They may be one of Europe's biggest spending clubs in recent years, but the absence of a natural striker to take Rashford's place shows that their recruitment strategy is deeply flawed, Neville argued.
"[Mason] Greenword, [Anthony] Martial and Rashford - all injured," he said during the game. "Lingard is now centre-forward for Manchester United.
"You look at that spend, over the last six or seven years, of £900m - just a little less than Manchester City - and this is what they've ended up with?
"It's incredible, £900m is a hell of a lot of money to have no centre-forwards."