Chelsea midfielder Deco had already been booked in the first-half for a foul on Roma's Francesco Totti when Spanish referee Luis Medina showed him the red card for his 80th minute attempt at taking a quick free kick.
Deco stormed: "The problem was the moment he said just wait, I touched the ball. It was crazy. I don't know why I was sent off.
"It was disappointing."
By the time Deco left the field, Chelsea were already 3-1 down and heading for a surprise defeat.
"We didn't play well and we made mistakes," admitted Deco. "But at the moment we are in first position and if we win the next game we will qualify.
"I don't know why we made so many mistakes, I hope it's just this one game. We need to be more organised.
"We want to continue playing in an attacking way. We are doing well and scoring many goals, but this was a bad game and now it's finished we have to think about the next one."