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United drew 0-0 with bitter rivals Manchester City on Saturday evening and their starting defence was Aaron Wan-Bissaka, Harry Maguire, Victor Lindelof and Luke Shaw.
Despite keeping a clean sheet, former United captain Keane claimed all four players plus de Gea aren't a title-winning unit.
He told Sky Sports per Metro: "We talk about trust in football, the back four and the goalkeeper are for me still the biggest problems [for Manchester United].
"Okay, they did well today but that's not a back-four that will take you places and that's a big concern.
"You just don't know what you're going to get from Man United right now. That's not a good sign when you're not sure what you're going to get from the team.
"Can you really trust this group to go on a run? The answer is no."
The Red Devils created little against Pep Guardiola's side and only managed to have two shots on target.
Despite this, manager Ole Gunnar Solskjaer was pleased with the performance and said it was United's best against City during his time in charge.
"Possession on the ball, control [was better]," the Norwegian said per Football365. "Of course I think last year at the Etihad we could have blown them away from the 10th to the 25th minute. We could have scored four goals.
:speaking_head:"Really poor, we got 2 booking in a derby game, I have never seen so many hugs and chats, I don't get it"- Football Daily (@footballdaily) December 12, 2020
Roy Keane was shocked about the Manchester Derby today between both teams pic.twitter.com/PkzRxPBe32
"Some unbelievable counter-attacking football, but we had about 30 per cent possession and that second half we were hanging on.
"You felt there was less moments to hang on today and we had more of the ball.
"Maybe that's because they had more respect for us as well. I don't know, but I'd like to think we've developed that side of it.
"And then today we just didn't have enough to get those counter-attacks that we have."
Featured Image Credit: Sky Sports & PA Images
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