Former Manchester United striker Dimitar Berbatov has labelled their performance against Crystal Palace as awful, and has encouraged them to start making more signings.
Throughout the summer Ole Gunnar Solskjaer has been desperate to sign Jadon Sancho, but Dortmund have refused to budge on their £120m asking price.
So far, they have only recruited Donny van de Beek from Ajax while the likes of Chelsea, Man City, Arsenal, Liverpool, and Spurs have all improved their respective squads.
In their first game of the new Premier League season on Saturday, United were defeated 3-1 at home by Crystal Palace, with former Old Trafford winger Wilfried Zaha scoring two of the goals to add insult to injury.
The home side looked fatigued and bereft of ideas, which had led to a scathing review of the performance by Berbatov.
"I said Manchester United need to hit the ground running, but they did the exact opposite at the weekend. It was only the first game, but they looked awful - slow and no transition between the lines," he told Betfair.
"I don't remember many efforts on target, and all of that speaks volumes about the attacking prowess of the team. In the end, Palace won deservedly.
"There's no excuse for United, the performance just wasn't good enough and hopefully it will just be a first-game blip.
"If we just based it on the first game then United are miles off the speed of the other top teams in the league, so I hope it is just down to it being game one.
The Bulgarian thinks that, if they are to compete at the top of the table and for Champions League qualification this season, they need to sign a new central defender and potentially players in other positions, too.
"If it becomes a trend, it could be a very long season. I and others have big expectations for United this season. They need a centre-half, and probably further recruitment also to be competitive," Berbatov added.
"At the end of the day, if Crystal Palace can score three past you, you are in trouble."
Man Utd bounced back slightly from the loss with a League Cup victory over Luton on Tuesday. This weekend, they travel to Brighton for their second league match of the campaign and first away from home.