However, Wenger's young team rose to the challenge as a first-half brace from Bendtner - who missed a string of chances against Burnley here three days ago - set them on their way towards the last eight in a 5-0 victory.
Following a good spell by Porto at the start of the second half, Samir Nasri and Emmanuel Eboue netted fine goals before the young Dane slotted home a penalty in the final minute.
"It shows you how football can change very quickly," said Wenger.
"Nicklas had not lost the confidence, it remains relatively stable.
"He came back with good focus and the same belief - that is one of his strengths as well."
The result also broke Arsenal's unwelcome near 32-year run of failing to overturn a first leg deficit in a European tie since beating Hajduk Split in 1978.
"The statistics and history were against us, but that just shows those do not mean a lot if you have a team on the pitch who believes they can win the game," said Wenger.
The Gunners boss continued: "We played fluent football with a positive start and the early goals gave us the belief we needed.
"We suffered a bit when we did not control the game at the start of the second half when we were not focused enough.
"Overall, though, we were good to watch and won with style."
Wenger rates Fabregas a only 50-50 to recover from his hamstring problem in time for Saturday's Barclays Premier League trip to Hull.
In Nasri, Wenger feels Arsenal have a fine asset, the France international capping a good night's work with a brilliant individual strike, turning three defenders before drilling in from a tight angle.
"What Nasri has shown in the game, he has shown in training and I have expected that to come out," said Wenger.
"He has talent and now he starts to be efficient - great players can assist and score goals, he is starting to do that now."
Arsenal could be joined in the quarter-finals by fellow English sides Manchester United and Chelsea.
While drawing either would be a chance for a slice of revenge given the Gunners' drubbings in the Premier League, Wenger does not want to make too big an issue of whom they may face.
The Arsenal manager said: "We do not choose Manchester United or Chelsea. If say we absolutely do not want to play an English team, we put ourselves in an inferior position at the start.
"Maybe it is a good opportunity, if we get them, to show we can do well.
"I do not want to say absolutely no English team.
"If I have a choice, I would say yes, but I do not have a choice.
"If we get them, we cannot do worse than we did in the championship - we can only do better.
"We will take the draw we get and also we do not have to make a negative obsession of not playing against Chelsea or Manchester United."
Wenger continued: "We are happy to be in the last eight - it is never easy to be in the top eight in Europe.
"The negative side is you only have good teams now - Barcelona, Manchester United, Chelsea, so it will be difficult.
"At the beginning of the season, nobody expected this team to be where we are, but we showed we have mental strength, we have good desire and good quality.
"I believe it is more important on our side to continue to improve - there are still some weak moments in the game we can deal better with, so before we talk about Barcelona or Manchester United it is important we continue to improve, then we have a chance."
Porto coach Jesualdo Ferreira felt his side had been somewhat unfortunate at key moments in the match.
Nasri cleared a header from substitute Cristian Rodriguez off the line, which would have made it 2-1, before going up the other end and scoring his fine individual effort.
"Arsenal turned the game around, put us under pressure and scored an early goal, that was the worst scenario we could imagine," Ferreira said.
"They controlled the match, but Porto deserved a bit more than we got.
"There were a couple of mistakes which were decisive. You cannot do that against a team of such quality."
Champions League 2009/10 Win Outright: Arsenal 8/1