"It's very frustrating, the referee was unbelievable," he told MUTV. "He didn't want to give the penalty, he didn't want to give yellow cards - I'm very disappointed.
"Refs don't protect skilful players. I think about the Arsenal player (Eduardo) and I'm scared sometimes to do skills because some players do unbelievable fouls and the ref protects the defender, not the skilled player! It's very disappointing and I'm thinking a lot about changing my game. When referees don't give penalties, yellow cards or reds, it's difficult to play."
Sir Alex Ferguson was left aghast that referee Martin Atkinson did not award a penalty for Sylvain Distin's clear bodycheck on Ronaldo in the seventh minute. Ronaldo added: "He (Distin) didn't try to take the ball. It's a joke."
But it wasn't just refereeing decisions that disappointed the 22-year-old. "We lost today and everyone is disappointed because we were the best team," he said. "We created ten chances while Portsmouth created just one. It's frustrating to lose like that.
"We controlled the game, but we didn't score. Carrick had one cleared off the line, Patrice hit the post... we had a great chance to win the FA Cup but we need to think now about the Premier League and the Champions League. These are the priorities."