They have a better chance without Totti IMO. He probably wouldn't have done much anyway. The team will however see his absence as a big loss and can be demoralised, or than can be motivated and individually give their all and work as a real unit. I don't rate Vucinic that highly technically, but be sure he's gonna run his ass off, and pressure the Man U defence whole game, whereas Totti might be invisible depending on whether things are going Roma's way or not. Most importantly is De Rossi will be the captain. He is the real leader of the team, much more than Totti. Plus, with that extra responsibility, he will look to lead by example and will be a monster.