Mourinho is bidding to win the competition for a third club, having taken Porto and Inter Milan all the way, but feels the Londoners who surprisingly won it in 2011 under Roberto Di Matteo are outsiders to reach the final in Lisbon.
"I think our chances this season, which is my first season (back) are not comparable with the chances of other clubs," Mourinho, whose side need a win to guarantee top spot having already qualified for the last 16, told reporters on Tuesday.
"Other clubs have stability in the squad which we don't have and the evolution of the team to be considered favourites to win the competition. But we have reached the last 16 and if we reach the quarter-finals I always used to say that every team in the quarter-finals has a chance to win it."
"My analysis of the situation is we are not favourites to win it. Bayern Munich are the favourites, they have unbelievable stability. Barcelona and Real Madrid too I will say clearly in terms of potential are clearly in front."
Mourinho confirmed that England left back Ashley Cole will return to the starting line-up against Steaua after being left out in favour of Cesar Azpilicueta in recent weeks.
"When he is on the bench, I can feel his will to win, even if he is not on the pitch," Mourinho said. "He is a fantastic guy, full respect from me and the players and he starts tomorrow."
Having said his side were "in trouble" after a 3-2 Premier League loss at Stoke City at the weekend, Mourinho was in better humour on Tuesday as he put his side's leaky defence, which also conceded three at Sunderland last week, down to bad luck.
"If we are in trouble, there are 17 teams in more trouble than us," he said. "We are creating a lot, we are playing a lot of nice football, but we have to finish off opponents.
"In this moment if you train defensive corners against the dummies, I think one dummy will score. All the rebounds go to the opponents, so I think they will go to the dummies too."
While Steaua will definitely finish bottom of the group, Mourinho is expecting a tough test from a side that twice drew with Basel and once with Schalke.
"(Steaua) were very important in the group phase because they took four points from Basel and they also drew with Schalke," he said.
"They are good friends because they took points from Basel and Schalke but to be perfect friends they have to lose tomorrow."
With a Premier League derby against Crystal Palace on Saturday and a League Cup quarter-final against Sunderland next Tuesday, Mourinho said he would rest a few players against Steaua whom they beat 4-0 earlier in the group.
"I don't want to call it rotation because when you say rotate comes with an idea that the team is weak," said the manager. "But I won't play the same team as the last match because some players need a little rest."
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