Mourinho's mood imitated the stormy weather in the Romanian capital as the Portuguese shortened his pre-match media conference to stride out to oversee the final training session under the roof at the National Arena.
The self-proclaimed Special One declared himself the Happy One on his return to Stamford Bridge and he has been accommodating, jovial and cordial at his pre-match inquisitions thus far.
But with the Blues looking to bounce back from the opening round loss at home to Basle, Mourinho showed the first signs of strain in taking exception to a question about absent forward Kevin de Bruyne.
With Juan Mata told he will start in Bucharest after beginning the season as a peripheral figure, De Bruyne was told he was not required and was left at home alongside his fellow Belgian Eden Hazard, who has an ankle injury.
"This is fantastic because nobody asks me about Mata," Mourinho said.
"You are for three weeks speaking about Mata, and now you are speaking about Kevin de Bruyne.
"So you are not interested in the players that are playing? You are interested in the players that are not playing.
"He was not selected. It was my decision. Only 11 can play and 18 can be selected.
"I try to decide by what they do on the pitch when they play and what they do in training.
"I try to be honest with Juan and say that he's going to play tomorrow, because of what he did in training and because of what he did in the matches he played.
"With Kevin it's the same. He's not selected because I didn't like the match he played against Swindon and I didn't like the way he was training.
"But you have this tendency to only ask about the guys that are not selected.
"See you tomorrow."
Mourinho anticipates a challenging evening at a venue where Rafael Benitez's side lost 1-0 in the first leg of a Europa League last-16 tie last term.
But the Portuguese believes that defeat, overturned at Stamford Bridge, will be an advantage.
"It's good for me," he said. "They lost, they know why they lost, they know it's not easy.
"Last season maybe was a bit of a surprise and this season they know they are going to play against a good team."
Mourinho expects Hazard to miss Sunday's Barclays Premier League match at Norwich and to next likely to be available for Chelsea after the international break, when striker Fernando Torres will next play domestically.
The Spain striker faces a possible four-match ban for his altercation with Tottenham's Jan Vertonghen during Saturday's London derby, when it appeared Torres scratched the defender on the face.
The Football Association is gathering evidence and has until 6pm on Tuesday to determine whether to punish Torres - who will be banned for one game anyway due to his dismissal for two yellow cards - based on video evidence.
Mourinho's hasty exit meant it was left to midfielder Frank Lampard to field questions on the prospect of Torres receiving a four-match ban.
"I'm not sure if he will," Lampard said.
"If he does, then we lose a player in good form, one of our main strikers for four games. People will have to step up."
Torres will be available to play a Steaua side who have lost only once in eight European home matches against English opposition and are undefeated in the league this season, winning six of their opening seven fixtures, scoring 19 goals in the process.
Lampard insists there will be no complacency on this occasion, with Chelsea knowing what to expect after experiencing a team roared on by a raucous crowd in March.
"You can never underestimate any team in the Champions League," Lampard said. "In the first game we didn't perform well, we lost. That's what happens.
"(But) we've got five games now to go, five games to put that right. That's a lot.
"We don't expect an easy game. We're lucky to have played Steaua Bucharest last season so we're fully aware of the good team they are. Last year we started slowly and we lost the game in the first half.
"Considering the position we're in now - we need a result tomorrow - we have to start the game at a top level.
"If we turn up, think 'we're Chelsea, we're going to come and play and win the game easily', that's not going to happen.
"It's down to us."