Manchester United should target a defender similar to club legend Rio Ferdinand or face a "long season", according to Dimitar Berbatov.
The Red Devils kicked off their Premier League campaign with a shock 3-1 home defeat to Crystal Palace and looked in desperate need of reinforcements.
Ole Gunnar Solskjaer has reiterated his desire to strengthen the squad with Jadon Sancho his top target, but so far only Donny van de Beek has arrived from Ajax.
United spent £130million on Harry Maguire and Aaron Wan-Bissaka in Solskjaer's first summer in charge, but former striker Berbatov feels more investment is required if the club are to be successful this season.
And after witnessing his formidable partnership with Nemanja Vidic's during his four years at the club, the Bulgarian says recruiting a defender in the style of Ferdinand would enable them to be competitive.
"If we just based it on the first game then United are miles off the speed of the other top teams in the league," Berbatov told Betfair.
"I hope it is just down to it being game one. 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. At the end of the day, if Crystal Palace can score three past you, you are in trouble."
Berbatov warned that the longer United delay making a signing, the harder it will be to attract the right player.
"They have less than two weeks until the window closes, and they can play some more games to figure out what they need in that time, but it's not easy," he added.
In Pictures | Luton vs Manchester United | 22/09/2020
"It can be hard to sign players - maybe the players don't want to come, it is hard to say - but their defence is what they need to work on.
"The goals they conceded at the weekend were poor defensive goals. If they don't strengthen up, it will be a long season for them.
"They need someone alongside Harry Maguire, look at how impressive he has been since he came in, and he is now captain. They need someone who is fast, like Rio Ferdinand was for Nemanja Vidic."
