Rosetti booked three Villarreal players and showed a straight red card to left-back Joan Capdevila for a high tackle on Ronaldo.
"The referee was very fair and strict with both teams," Fergie said.
"Capdevila may not have got that red card in England, so there is a different interpretation of the physical aspect of the game in European matches.
"When you see the tackle again, it was very high. When you go as high as that you risk a red card.
"There were three tackles on Cristiano and I think the referee has quite rightly booked the players and then dished out a red card."