Pep Guardiola's men have shone at times this season, but have also missed a clinical finisher as they failed to score at home to Southampton and Crystal Palace to fall five points behind Premier League leaders Chelsea.
United are three points further back after a run of one win in five league games.
The Red Devils have won in just five of Ronaldo's 12 appearances, while the likes of Jadon Sancho, Marcus Rashford and Mason Greenwood have been marginalised.
A 3-0 win at Tottenham last weekend bought Ole Gunnar Solskjaer some time as United boss.
But the defensive frailties that Liverpool exposed in a 5-0 humiliation of Solskjaer's men less than two weeks ago were on show once more in Italy on Tuesday, especially after Raphael Varane limped off with a hamstring injury.
Varane has been ruled out for a month leaving Solskjaer with a headache of how to protect his remaining sluggish centre-backs.
CAN CONTE TRANSFORM TOTTENHAM?
Antonio Conte makes his Premier League return on Sunday when Tottenham travel to Everton hoping to get back in the race for a top-four finish.
Spurs swooped for the former Chelsea boss, who had been linked with the United job, within 24 hours of sacking Nuno Espirito Santo on Monday.
Hiring one of Europe's most coveted coaches is a sign of Spurs' ambition to not slip back into mid-table mediocrity after the highs of Mauricio Pochettino's five years in charge between 2014 and 2019.
Conte needs to get Harry Kane firing, with the England captain having scored just once in the Premier League this season.
But Everton could prove perfect opponents to kickstart the Italian's reign after losing their last three games.
NEWCASTLE'S NEW DAWN IN DISARRAY
Spurs' swift move for Conte highlighted a first month to forget for Newcastle's new owners.
Three weeks after sacking Steve Bruce, the Magpies are still searching for a new boss after being turned down by Villarreal coach Unai Emery.
Former Bournemouth manager Eddie Howe is the new frontrunner, but caretaker boss Graeme Jones looks set to take charge for a third game at Brighton on Saturday.
Backed by the Saudi sovereign wealth fund, Newcastle will have huge backing to reinforce in the January transfer window.
But they badly need an overhaul just to avoid relegation.
They sit second bottom of the table on four points, without a win in their opening 10 Premier League games of the season. (Text, excluding headline, courtesy of AFP)