United will be without Antonio Valencia for four weeks with a hamstring strain picked up in the 2-0 win at Ajax while City are to lodge an official complaint after striker Mario Balotelli allegedly suffered racial abuse from the crowd during last night's win at Porto.
The club informed Uefa's matchday delegate of their intention to complain after the 2-1 Europa League success at the Estadio Dragao and will follow it up in writing today. There were suspected outbreaks of monkey chanting aimed at Balotelli and Yaya Toure during the first half but it appeared most noticeable when the former was substituted 12 minutes from time.
Manager Roberto Mancini said immediately after the game he had not heard it but Balotelli later informed a club official he felt he had been abused.
A club representative then told media a complaint would be made. City midfielder Toure also backed up Balotelli's claim by saying he "heard something" from the stands.
The midfielder said: "That's why we all like the Premier League, because it never happens there. Maybe in foreign countries they don't expect black players. I think in future it will be okay, they will change their minds and the game will become more open. Football is football and the fans come to enjoy the game, and we don't want things like this to happen. Mario is now able to control things like this and that is very important for the club."
The matter made for a sour end to first leg of the last-32 contest and overshadowed a fine comeback by City.
The Premier League leaders trailed to the Portuguese champions at the interval after a 27th-minute strike from Silvestre Varela.
An own goal by Alvaro Pereira, who had been under pressure from Balotelli, gifted them an equaliser after 55 minutes and substitute Sergio Aguero snatched victory six minutes from time.
City will now be confident of securing a place in the last 16 in the return leg at the Etihad Stadium on Wednesday.
Mancini said: "I think it was a good result because it is difficult to win here. I think the guys were fantastic. I think we are 60 per cent to go to the next stage but Porto is a top team. It will also be a difficult game in Manchester. We should keep the same concentration we had tonight."
United manager Sir Alex Ferguson admitted it is "a bad blow" to lose Valencia for a month through injury. The South American had only been on the field for 10 minutes following his introduction as a second-half substitute.
However, after playing an integral role in Javier Hernandez's second in United's 2-0 Europa League win in Amsterdam, Valencia was immediately replaced.
And Ferguson confirmed the news was grim. "It is a bad blow for us," he said. "He has a hamstring injury unfortunately. It will be four weeks."