But it is one wager he will be more than happy to lose if the Romanian champions can deliver another blow to Rangers' European hopes when the teams meet again in Bucharest on Wednesday night.
The game is one Walter Smith's men cannot afford to lose if they are to retain any hope of progressing from the group stages but, as far as Petrescu is concerned, a repeat of Unirea's exploits in Glasgow a fortnight ago are extremely unlikely.
"I bet that they will not take any more points because Romanian teams always win a game and after that they lose the games," he said.
"I think this time will be exactly the same because this is the mentality of this country.
"Winning tomorrow would be a miracle. I don't even remember the last time that a Romanian team won two consecutive Champions League games.
"The media and everybody who believes that tomorrow will be an easy win for Unirea are not right. I will see tomorrow after the game if they win.
"I hope I lose - it is the first bet in my life that I want to lose. I hope they win."
The players head into the game in more confident mood, with Serban Varga declaring: "I hope that all of us take his money tomorrow after the game."
When pointed out to him no Romanian team has won a Champions League match at home for 13 years, Varga replied: "I know this and it would be great for the morale if we won tomorrow.
"We haven't had very good games, especially domestically where we haven't managed to play well."
Despite claiming to have a lack of confidence in his players' ability to claim another three points against the Scottish champions, Petrescu is well aware of what is at stake.
"It's the most important game of my coaching career," said the former Chelsea star.
"I wasn't thinking that in matchday four I would have the opportunity to play such an important game. I was hoping maybe in the last game we would have a chance so this is great.
"For them, it's a big game because if they lose tomorrow they are out of the competition. It's important for them but, for me, it's more important than for Rangers.
"They are used to these games and they play every year in the UEFA Cup or the Champions League. This is our first time in the Champions League, it's a big game and we hope to win.
"I don't think a draw is good enough for Rangers but maybe a draw is good for us. If we draw, we have a 90% chance of going into the Europa League, which is also a good achievement for us. They have to win if they want to qualify."
Rangers had enjoyed an ideal start to the last game when a Pedro Mendes effort was deflected into the back of his own net by Ricardo Vilana after just two minutes before the hosts collapsed in the second half.
But, according to Petrescu, his players were not at their best on the night despite the comprehensive win.
He continued: "I said to them that I think we didn't play that well. We scored from three free-kicks and in open play we could have done better.
"We know that it will be a completely different game to the one in Glasgow, they won't treat us the way they did in that game.
"We know they were hurt after that match because I have read the Scottish press and I know they are here for revenge and they want a very high score.
"They will be more aggressive and offensive than in the game in Glasgow and will come to attack. Any result is possible in the game tomorrow."
Unirea are likely to be without Bruno Fernandes after he limped out of their game against Craiova with a thigh injury.