Darren Fletcher has warned Manchester United they have to improve their defensive record and stop conceding goals at set-pieces if they are to win Sunday's Manchester derby.
The Premier League leaders beat Reading 4-3 on Saturday, the seventh time they have come from behind to win already this season, to go three points clear of Manchester City.
But United have now conceded 21 goals, almost twice as many as their neighbours and three more than relegation-threatened Sunderland.
Reading scored twice from corners, with Adam le Fondre and Sean Morrison heading in Nicky Shorey's deliveries, to compound United's problems defending dead-ball situations.
United lost to a goal from a corner, headed in by Vincent Kompany, on their last trip to City in April and midfielder Fletcher believes they have to improve at the back.
"We just can't keep conceding goals," he said. "We're concerned about it, we don't want it to happen, it's just individual mistakes, particularly at set-pieces.
"But we keep responding, we keep coming back, and that's a good attribute to have. Going against teams like City, they will definitely punish any slack defending, so we're going to have to be on top of our game.
"But, sometimes, you can really focus on the bigger matches and make sure it doesn't happen."
City won the Premier League title on goal difference from United last season, aided by two victories against their neighbours.
"The derbies were a major factor in that," Fletcher added. "So we don't want to give City any advantage by them winning against us. I think it's about time we got a result against them."