Rio Ferdinand is concerned Manchester United will struggle to compete with Manchester City at Old Trafford on Saturday afternoon.
The Red Devils were fortunate to escape with a 2-2 draw against Atalanta on Tuesday evening, with Cristiano Ronaldo scoring a last-minute equaliser.
And the club's former captain Ferdinand believes they will get "slapped" against City if they repeat that performance in the Premier League.
He told BT Sport: "They can't have that performance against City. If they do, they'll get slapped by three, four, five goals again because that shows a real slack opposition being all over the place."
United have picked up just four points from their last four Premier League games, suffering an embarrassing 5-0 home defeat to Liverpool last month.
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Manager Ole Gunnar Solskjaer is under huge pressure at the moment and needs some big results over the coming weeks to restore faith in his leadership.
And Ferdinand is concerned about the lack of intensity the Norwegian's team showed against Atalanta in Bergamo on Tuesday.
The retired defender insisted: "The intensity wasn't there. It was disarray, it was just hope and it comes down to a moment of brilliance by Ronaldo again and he saved them that half."
Atalanta opened the scoring against United in the 12th minute when Josip Ilicic squeezed a shot underneath David de Gea.
Ronaldo equalised just before half-time and needed to repeat the trick later on after Duvan Zapata restored the hosts' lead in the 56th minute.
Raphael Varane was forced off with a tight hamstring during the first half and manager Solskjaer will be hoping the defender's injury isn't serious.
Ferdinand continued: "We saw the importance when Varane is there, he covers both sides very well and there's a sense of security.
"As soon as he went off they looked so shaky and when they went to a 4 they looked like they are easy to play against and they were fortunate that the main man Ronaldo stepped up."
Discussing Ronaldo's performance, Ferdinand summarised: "He don't work hard enough he don't press but he puts the ball back into the net… but puts the ball in the back of the net.
"The hardest bit about the game, everyone tries to be that guy in the critical moment.
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"He steps up any time, any place; the biggest moments, he's there. There aren't any more words we can say about him that justify the way he provides in the biggest moments."
And fellow United legend Paul Scholes added: "This is what he's been doing for ten, 15 years.
"It's something we expect and again he's pulled his team out of a really poor second half performance. It's a brilliant finish its instinct and pure technique what more can you say about this lad."
