Rio Ferdinand was unimpressed with Manchester United's performance against Atalanta last night. The Red Devils drew 2-2 against the Serie A team in a Champions League group stage encounter.The Red Devils will next face Manchester City in the Premier League on Saturday.Speaking about Manchester United's performance after the Atalanta match, Ferdinand 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."Manchester United went behind twice in the match and were rescued on both occasions by the brilliance of Cristiano Ronaldo. Atalanta, despite missing several key first-team players, seemed to be the more dominant team in the game.Manchester United@ManUtdWe fight until the end, and we couldn't do it without your incredible support 👏#MUFC | #UCL twitter.com/ManUtd/status/…Manchester United@ManUtd🔴 𝐑𝐄𝐃 𝐀𝐑𝐌𝐘! 🔴#MUFC | #UCL5:37 AM · Nov 3, 20215580468🔴 𝐑𝐄𝐃 𝐀𝐑𝐌𝐘! 🔴#MUFC | #UCL https://t.co/jYv3NeGlwMWe fight until the end, and we couldn't do it without your incredible support 👏#MUFC | #UCL twitter.com/ManUtd/status/… https://t.co/WrHoSr1UqtManchester United, playing in their new 3-5-2 formation, were unable to retain possession or make pressing runs into the opposition box. In short, they were outplayed for the majority of the match.Speaking about the overall performance, the former Manchester United 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."Football on BT Sport@btsportfootball"This is what he does! It's illegal! Alien!"We think @rioferdy5 enjoyed Cristiano Ronaldo's goals against Atalanta tonight... 😂#UCLTONIGHT4:59 AM · Nov 3, 20214855781"This is what he does! It's illegal! Alien!"We think @rioferdy5 enjoyed Cristiano Ronaldo's goals against Atalanta tonight... 😂#UCLTONIGHT https://t.co/8NlRaF4qWuManchester United will be concerned with Raphael Varane's injuryManchester United defender Raphael Varane was taken off after the first half last night. He pulled his hamstring during the match and it will be a key concern for Ole Gunnar Solskjaer ahead of their crucial encounter against Manchester City at the weekend.Ferdinand said:"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."Raphael Varane joined from Real Madrid this summer to partner Harry Maguire at the heart of Manchester United's defense. The Frenchman has made an impressive start to his United career and has already become their best defender on the pitch.Ole Gunnar Solskjaer's job is already on the line and the Norwegian will be worried by the extent of Varane's injury. Should Manchester United suffer another big defeat to Manchester City this weekend, Solskjaer might just get the sack at Old Trafford.If Varane is unable to play against Manchester City. Solskjaer will have to use Maguire, Eric Bailly and Victor Lindelof as a back-three. Pep Guardiola's men will be relishing the opportunity since none of the aforementioned defenders are in great form at the moment.