Manchester United boss Ole Gunnar Solskjaer expressed his frustration at the Football Association's scheduling after local rivals Manchester City were handed an extra day to recover ahead of Sunday's derby.
United eased past Derby County 3-0 in the FA Cup fifth round on Thursday, but City have had an extra day to prepare for their Premier League trip to Old Trafford after beating Sheffield Wednesday a day earlier.
"It's that help we get from the FA again, that extra 24 hours that they've (City) had. I cannot believe why there's a derby on Sunday and we've got to play Thursday night," Solskjaer told a news conference.
"What's the point in that? That doesn't level the playing field."
City also had an extra day to prepare ahead of December's derby at the Etihad Stadium, although goals from Marcus Rashford and Anthony Martial secured a 2-1 win for United.
"It was the same when we beat them last time to be fair, we played Wednesday and they played Tuesday. Those 24 hours are important so we've got to be good at recovering now."
United are unbeaten in nine games across all competitions after Thursday's win over Derby.
Meanwhile Guardiola's City have won their last five matches, including a Wembley final against Aston Villa for their third straight League Cup title.
"They'll feel confident," Solskjaer added. "They've hit form, they're playing well. They've just been at the Bernabeu winning there (against Real Madrid), they've won the cup final so I'm sure they'll come confident."
United captain Harry Maguire was unavailable against Derby with an ankle injury, and now faces a race to be fit to face City in a crucial match for United's Champions League qualification hopes.
"Well, he's touch and go because he rolled his ankle but, hopefully, he'll recover quickly," Solskjaer said.
"I wasn't going to rest him (against Derby), I didn't give him a day off birthday-wise but I hope he'll be fit."