The opportunity for Manchester City's supporters was too good to pass up. Their team were on their way to a 4-0 win against Porto and the opposition had a striker going by the name of Hulk who was showing signs of frustration. "You're not incredible," the home fans sang.

Hulk, real name Givanildo Vieira de Souza, certainly did not look like he had superhuman powers as Sergio Agüero inspired City to a 4-0 victory to send them into the next round of the Europa League with an aggregate 6-1 victory.

Porto, however, have not seen the funny side. The Portuguese club are now preparing an official complaint in which they will ask Uefa to open disciplinary charges against the Premier League leaders – and they are denying it has anything to do with an apparent humour bypass.

Rui Cerqueira, the club's spokesman, said the matter would be reported on the grounds that City's supporters had acted in an improper way. Hulk had also been targeted with chants of "cheat" because of the Brazilian's tendency to look for free-kicks and Porto will claim he was the victim of unsporting behaviour that warrants an official investigation.

The allegations have been met with bewilderment at the Etihad Stadium where they could be forgiven for wondering if it is tit-for-tat retaliation after what happened when the two teams met at the Estádio do Dragão last week. Uefa has opened a disciplinary case after two of City's black players, Mario Balotelli and Yaya Touré, were racially abused – just as Didier Drogba and William Gallas were when they played there for Chelsea in 2004.

Porto, however, have made it clear they are outraged by City lodging a complaint. The Porto version of events that it was actually both sets of supporters chanting in support of Hulk and "Kun" Agüero. "Kun, Kun, Kun; Hulk, Hulk, Hulk," Cerqueira said at the time, demonstrating the chants. "These can be easily confused with racist chants."

All of which was a strange explanation given that Agüero was not even on the pitch at the time. Incredible, you could say.