Pellegrini's men slumped to a third consecutive away league defeat - and fourth in all competitions in a row on the road - after losing 2-1 at Crystal Palace on Monday to leave their title defence in tatters.
City were level on points with leaders Chelsea on New Year's Day, but four wins and as many losses over the 11 subsequent league games means they head to Old Trafford a point and a place behind United in fourth.
But Pellegrini insists his team are still doing plenty of good work on the field, having seen them dominate for long periods without reward at Selhurst Park last time out.
"We lost the last three [away league] games, especially the last two with two goals in set-pieces, against Burnley and Palace," he said.
"We hope we continue keeping the good things we are doing because we are not a disaster, we're not a mess. We are doing a lot of things well."
City captain Vincent Kompany is facing a late fitness test on a hamstring injury sustained during the second half against Palace, while forward duo Wilfried Bony (ankle/knee) and Stevan Jovetic (muscle strain) remain injured.
The latter concerns could mean a start for Edin Dzeko alongside Sergio Aguero in attack. The Bosnian striker has just four Premier League goals this season - the same number he has netted on his past two appearances at Old Trafford on derby day.
Indeed, City can be buoyed by impressive recent form against their neighbours, having won the past four derbies and six of the last seven in the league. A victory at the weekend would make City the first team to win five consecutive Premier League games against United.
But Louis van Gaal and his squad can be buoyed by a haul of 40 points on their own turf this term - the highest number in the Premier League, where they are on a winning run of five matches.
Nevertheless, the Dutchman is keen to keep his players on their toes.
"'Never change a winning team' is an English expression," he told MUTV. "You must see the quality of your opponent and make a decision with the staff, so it's not always [about] never changing the team."
Robin van Persie has returned to training for United following an ankle injury, but the 169th Manchester derby comes too soon for him, while England defenders Chris Smalling (illness) and Luke Shaw (hamstring) will be assessed.
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