Going into the 19th match of the campaign on Wednesday against Newcastle, United has already conceded 25 times, just eight short of the total in 38 games last year when the title was lost on goal difference.
As they prepare for Newcastle's visit, United's players have been recalling how they produced just one of three clean sheets so far, in a 3-0 victory at the northeast club in October.
"We went there with the right mindset and defended well, we had great shape and got two early goals," United defender Jonny Evans said. "It was probably one of our best defensive performances of the season."
United is four points ahead of City, but the second-place team's defence has only been breached 15 times so far in its title defence.
United's firepower up front, though, is more lethal, hitting 44 goals in 18 matches compared with 34 by City.
United have become reliant on Robin Van Persie, who has scored 12 goals in the league alone since joining from Arsenal in August.
"I reckon we got our Christmas present early - right at the start of the season when Robin van Persie arrived at Old Trafford," manager Alex Ferguson said. "I am reluctant to subscribe to the cult of the individual but sometimes there really is a situation when you are lucky enough to find the last piece of the jigsaw.
"As we approach the halfway point of the season, I have no hesitation saying that he has made a vital difference to our chances of making our mark in this season's title race ... he has moved us forward as a team."
But Ferguson has a quandary up front as both Van Persie and Wayne Rooney are one booking away from suspension.
Rooney, though, was ineffective in Sunday's 1-1 draw at Swansea, and Javier Hernandez and Danny Welbeck will both be hoping to regain a starting spot on Wednesday.
Newcastle hasn't won at Old Trafford since 1972 and is enduring a gloomy season after a surprise fifth-place finish in May.
Languishing five points above the relegation zone in 14th place, Newcastle's victory over Queens Park Rangers on Saturday was only the team's second in 12 games.
"They've found it a bit tougher this year but I know they've had some injuries, which have played a part," Welbeck said. "You can still see potential throughout their team. They have some real attacking threats — everyone knows about players like Demba Ba and Papiss Cisse."
Having already lost to Man City this month, Newcastle follows up the trip to United with matches against fourth-place Arsenal on Saturday and fifth-place Everton on Wednesday.
"When you looked at the program at the start, you could see this was a tough run for us — and even tougher now with the results we've had," Newcastle manager Alan Pardew said.
But Pardew is upbeat.
"We've definitely picked up, we look a little bit more resilient, we picked up a very important clean sheet on Saturday," he said. "And I think we'll be in good heart for what is one of the toughest games of the program: Manchester United away."
Newcastle is just a point and a place above Sunderland, and its northeast rival hosts United's neighbour City on Wednesday.
City cut United's six-point lead to four at the weekend thanks to Gareth Barry's late winner against bottom-place Reading.
"We are a very strong team, self-motivated and full of confidence at the moment," City defender Matija Nastasic said. "What is important is that we keep the same attitude, if we want to win the Premier League."
So too does Chelsea, which is at Norwich on Wednesday.
Resurgent under Rafa Benitez, Chelsea's 8-0 victory over Aston Villa on Sunday put the side 11 points behind leader United with a game in hand.
"We have to sustain this run and it will be easier for me to say we can compete," Benitez said.
Fourth-place Arsenal was due to play West Ham on Wednesday but the game was postponed due to a strike by London Underground workers.
They are the only sides not in action on Boxing Day. So Arsenal could be overtaken by Everton, Tottenham or West Bromwich Albion, who are only behind Arsene Wenger's side on goal difference.
Everton hosts Wigan, Tottenham is at Aston Villa, and West Brom plays a Queens Park Rangers side that is just a point off the bottom of the standings.
Bottom-place Reading hosts Swansea, Southampton, which is only out of the drop zone on goal difference, is at Fulham, and Liverpool heads to Stoke.