Cristiano Ronaldo may be the most devastating attacking force in world football, but he is unlikely to win a popularity contest among his fellow professionals. The Manchester United forward was accused yesterday of showing contempt for his opponents during his team's 2-0 victory over AS Roma in the Champions League on Tuesday and felt compelled to defend himself as he denied allegations of showboating.

Ronaldo lived up to his reputation by scoring his 36th goal of the season during United's win in Italy, putting them on course for a semi-final against Barcelona, but David Pizarro, the Roma and Chile midfield player, complained that the 23-year-old had performed arrogantly.

While Ronaldo's talent continues to attract covetous glances from Real Madrid - whose coach, Bernd Schuster, said yesterday that the Spanish club will sign him “in a few years” - his extravagant dribbling style antagonises opponents, with Pizarro hinting that Roma's players plan to teach Ronaldo a lesson in the second leg at Old Trafford on Wednesday.

“There is no doubt that he has quality, but it is also true that he has a big head,” Pizarro said. “Some of his little tricks in the middle of the pitch were unnecessary and he needs to show some respect to his opposition. You can bet that we will have something to say about it in the return leg.”

Sir Alex Ferguson, the United manager, would argue that Pizarro has already had something to say about Ronaldo, having been booked for a lunge on the Portuguese late in the first half. Ronaldo had just scored the opening goal and, having been subjected to rough treatment, was enjoying himself with what some would call showboating. But he denied that his style was designed to humiliate the opposition.

“I have not been taking the mickey or playing the fool with anybody,” Ronaldo said. “On the field I respect my opponent. If someone thinks my moves are mockery, they are wrong. I play for the team, not to show off. If I have a certain attitude, it's simply because I have always had it and I will continue to do that.”

Ferguson has also been left with some explaining to do after he and Carlos Queiroz, his assistant, were charged with improper conduct in the wake of their remarks about Martin Atkinson, the referee, after their FA Cup sixth-round defeat by Portsmouth at Old Trafford on March 8.

Ferguson, who was unhappy that United had not been awarded a penalty for a challenge by Sylvain Distin on Ronaldo, complained that Atkinson had been “on Portsmouth's side” and also made comments about Keith Hackett, the general manager of Professional Game Match Officials Ltd, while Queiroz referred to Atkinson as “this robber”.

Both are expected to be fined. Ferguson is likely to incur the larger punishment, having served a touchline ban this season for making abusive remarks to Mark Clattenburg, the referee, during his team's defeat away to Bolton Wanderers in November, but a second ban is thought unlikely given that this charge relates to post-match comments rather than his behaviour on the touchline.

Ferguson was waiting last night to discover the extent of the injury suffered by Nemanja Vidic in Rome. The Serbia defender could not be fully examined yesterday because his left knee was too swollen to allow a proper scan.