Manchester United manager Jose Mourinho said he was "very disappointed" to see rivals City eliminated from the Champions League.
Pep Guardiola's City made a last-16 exit on Wednesday, a 3-1 defeat at Monaco seeing them go out on away goals after a 6-6 result on aggregate.
Of the Premier League's top six, only United remain in European competition, having reached the Europa League quarter-finals on Thursday.
Mourinho said City's defeat saddened him, as it made things even more difficult for United to secure a top-four finish.
"I am very disappointed [at City going out]. I am not making fun of it. It's bad for us for many reasons," he told BT Sport.
"It's bad for us as an English club and it's bad for us as all of the five teams in front of us in the Premier League don't have international football.
"They will be playing one match a week, like some of them have been doing since the beginning of the season.
"They don't know what fatigue is, they don't know what accumulation is, they don't know what it is like to go home and not sleep because of fatigue.
"We will fight to our limits - that's what we have to do."
Mourinho is unhappy about United's busy run of fixtures, their 1-0 win over Rostov on Thursday having come days after an FA Cup exit at Chelsea, with a league meeting against Middlesbrough to follow on Sunday.
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