Former Manchester United striker Dimitar Berbatov believes they need to buy defenders in January.
They have only kept three cleansheets in 17 matches this campaign. In contrast, United have scored in 14 games.
Berbatov told the Mirror: "They need to reinforce at the back especially centre backs, one or two.
"In every other position they have several quality players and everything they need, so just defence needs reinforcing. Hopefully they can find one or two and then they should be okay.
"It'll be more difficult in the winter transfer window then the summer one to make signings for Mourinho but he should be able to find one person he wants to bring in.
"So if there is someone he wants to bring in the board should listen to him because it's obvious that the club aren't perfect right now and aren't in the position that they want to be in. They need to improve on that, if it means bringing one or two people, it shouldn't be a problem.
"I mean it's not like Manchester United are short of money."