The Red Devils announced Erik ten Hag as the club's new manager shortly after the end of last season, with the hope of kicking on in their plans in the transfer market.
However, as it stands at the time of writing, United are yet to make any new signings despite the need to significantly strengthen their squad.
A host of players, including Paul Pogba, Nemanja Matic, Jesse Lingard, Juan Mata and Edinson Cavani, have already left the club, meaning that Ten Hag's first-team squad is looking pretty thin ahead of the new campaign.
Pressure is also being heaped on the club with rivals Man City, Liverpool and Tottenham all making significant headway in the market.
Pep Guardiola's side have added Erling Haaland to his squad, the Reds have signed Darwin Nunez, while Spurs have completed deals for Ivan Perisic and Fraser Forster.
With United needing to make up ground on their divisional rivals, club legend Neville believes his old club are behind schedule and fears for what it could mean for Ten Hag's preparation for the new season - with the fixtures now released.
Taking to social media, the former defender said: "I know it's early in the window but it's worrying that United are struggling to get business done.
"The others seem set and ready yet United can't get moving.
"Erik ten Hag needs his group together asap to mold them. Bringing them in late will only make it harder for him! [Fingers crossed] it happens soon!"
Despite a lack of progress in the market, United continue to be linked with moves for Ajax star Antony and Barcelona's Frenkie de Jong, with talkSPORT's Alex Crook claiming both players look like 'definite arrivals'.
The transfer reporter said: said: "I think United are definitely in for Antony.
"His numbers for much of the season were decent, I think he missed the end of the campaign injured but he's obviously someone that Erik ten Hag knows well.
"There's been talk of a £40million bid going in for him, certainly my sources at United haven't distanced themselves from those rumours so I think Antony is a target.
"In fact I've had a text from somebody else saying if he doesn't become a Manchester United player this summer I would be surprised.
"I think at the moment him and Frenkie De Jong look like two definite arrivals at Manchester United and are two players that Ten Hag clearly trusts and believes he can get the best out of."