The Daily Mail says words were exchanged between Nani and reserve team midfielder Petrucci after a training ground challenge at Carrington.
Nani later called the 20-year-old Italian over in the gym and the rest of the squad thought the pair would talk through their differences, but they were said to be stunned when Nani appeared to land a punch on Petrucci.
Other players quickly stepped in to prevent further trouble.
Ferguson is believed to be fuming over the incident, which took place earlier this week and will only harden his resolve to offload Nani at the first opportunity.
A £25million deal to sell him to Zenit St Petersburg in the summer fell through because his 'unreal' wage demands put off the Russian club.