If Rooney is booked during the game, and United progress to the final, he will be unable to play.
This would be a repeat of the situation the Red Devils experienced in their 1999 Champions League win against Bayern Munich, when both Paul Scholes and Roy Keane missed the game through suspension.
Red Devils boss Sir Alex Ferguson has stated that Rooney is working on his overly-aggressive nature, but admits his temper could get the better of him.
Speaking to News of the World, Ferguson said, "With Wayne it's simple. When a guy whacks him, he never argues, never worries and never retaliates.
"Most of the time, it's when he gets an injustice from the referee, or he gives the ball away and races after it to get it back. When he loses the ball - that's the only time I worry about him.
"You hope every player, once they get to their mid 20s, gets maturity and understanding and I don't think he'll be any different."
United left-back Patrice Evra is in the same boat as Rooney, and both players will have to watch their step on Tuesday at the Emirates Stadium.
Nick Turner, Goal.com