Nani said: "All of us have given him encouragement. He's been so instrumental for us all season and has won so many games for us.
"We'll be ready to knock them out at Old Trafford."
Wayne Rooney also backed Ronaldo to bounce back and show why many regard him as the world's best player.
Roon said: "He's an unbelievable player and I'm sure that the penalty miss won't affect him at all. He's scored 38 goals this season.
"He's missed a penalty, but it's difficult up front on his own and he did well. His football speaks for itself. All the great players have missed penalties so he won't worry about that."
United assistant-manager Carlos Queiroz added: "Missing penalties is part of the game but we were very happy with the way he played for the rest of the game.