Arsenal will be without half-a-dozen first-team regulars for Wednesday night's Group F clash, including England forward Theo Walcott, Ivory Coast striker Gervinho and centre-back Laurent Koscielny who have all picked up minor knocks after Saturday's 3-0 league win over Bolton.
Wenger, though, maintained there was no underlying problem in a packed Gunners treatment room.
Another headache: Wenger shows the strain as he works with his players ahead of their latest European clash
'I don't think that the medical staff made any mistake on this case,' said Wenger.
'It is unfortunate, we are all very sad at the club that it has happened to such a young boy.
'We were even more worried, but it was a big relief yesterday for everybody at the club that the operation went well.
'The fracture he had didn't heal, what I have been told is that we had to go in quite quickly to repair itand that's why we decided to do it so quickly.'
Training day: Aaron Ramsey leads his team-mates on a morning run at London Colney
The 19-year-old may not return to action untilFebruary after an operation to repair the right ankle he damaged in pre-season.
Wilshere first collected the problem a stress fracture while on England duty in the 2-2 draw with Switzerland at Wembley in June butquestions were raised over the way his return from injury was managed.
But Wenger denied any suggestions hisstaff were responsible as Arsenal prepared for their Emirates meeting with the Greek outfit.
'He had an earlier fracture at the age of 16 that healed and around that he created another opening,' Wenger added.
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'Is it down to a kick, is it down to playing too much? I don't know, I am not a specialist, but that is unfortunately what happened. He told me he felt it the first time [when playing for England] against Switzerland.'
Despite the loss of Wilshere, Wenger believes other youngsters such as Emmanuel Frimpong, can now step up to the challenge.
'It makes us a bit shorter in midfield and it puts a bit more pressure on our midfielders to perform game-in, game-out, because we will have less rotation possibilities, butevery club can be tested on that front so it shouldn't impact our end result of the season and shouldn't be any excuse at all,' he said.
'If [Abou] Diaby comes back well and quickly we should, number wise, quality wise, be all right.
'That depends as well on the injuries we have in the next two or three months.
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'Touch wood we can absorb the number of games we face well and we have some younger players pushing like [Francis] Coquelin like Frimpong, who are playing very well, so hopefully we can cope with it.'
Arsenal eased the pressure with theirPremier League victory over 10-man Bolton on Saturday after what has been one of the most testing spells of Wenger's 15-year tenure.
In Europe, though, the Gunners have produced the goods, securing a place in the group stages with an impressive two-legged win over Serie A pacesetters Udinese and then coming away from Dortmund with a 1-1 draw after a late goal salvaged a share of the spoils for the hosts.
Wenger, however, is not expecting any easy ride against the Greeks, where he will serve the final match of his touchline ban.
'We are a train that started a bit late in the station, so we cannot afford to drop our consistency,' he said.
Talking tactics: Gunners boss Arsene Wenger talks to his team ahead of Wednesday's Champions League clash with Olympiacos
'You never win easy in the Champions League - when you give absolutely everything, you have a chance to win.
'If you do not prepare properly, then you have no chance.
'After 14 consecutive years in the Champions League, if we have not learned that, then we are really stupid.'
For Arsenal midfielder Mikel Arteta, this season is a first taste of top-flight European action following hisdeadline-day switch from Everton.
The Spanish midfielder, 29, is relishing the challenge.
He said: 'It is hard to explain what it is about the Champions League, the atmosphere is different you are playing against the best teams in Europe every time.
Thumbs: But Jack Wilshere could be out for five months after the young England ace had surgery on his troublesome ankle
'You are expected to win here, which is a really good thing.
'We need to be prepared for that and have to take things game-by-game. We all know we have the ability to do it.'
Per Mertesacker insists Arsenal's defence is tightening up as Wenger attempts to deal with his toughest time at the club.
'It worked and on Saturday you saw we had a better defence. We were closer together.
'After the four goals against Blackburn we lost a little bit of confidence but we were very proud of our performance on Saturday. We want to go on with this way of defending.
'I want to do a good job, especially in the defence. That is my role. If the coach praises me then that is a great thing but I have to go on and keep on working. My performances canimprove.'
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