The France international was a shock inclusion in the 3-1 defeat to the Red Devils after Wenger had categorically stated he would miss the game after limping out of the midweek victory over Huddersfield at half-time.
Instead, Lacazette was named in an Arsenal side that slipped to a first defeat at the Emirates Stadium since March and scored the consolation goal as Jose Mourinho's United left north London with all three points.
Mourinho made a pointed jibe towards Wenger when asked about his team news ahead of Sunday's derby against early Premier League pace-setters Manchester City.
"I'm telling the truth," Mourinho said when listing the availability of some of his players
"It's the truth, eh? No stories of Lacazette or (Manchester City midfielder) David Silva
All the truth."
Wenger, speaking ahead of Arsenal's final Europa League group game at home to BATE Borisov, was asked about Mourinho's comments and said he was genuinely expecting to go into the United game without his club-record signing.
"I thought he wouldn't play," he said
"In the end, he could play
What did you want me to do, leave him at home?
"I'm always honest
I thought really that he wouldn't play, that's what I had been told
"After, he had a test on Friday morning and went to the game
We weren't sure he would play but he tried and he played, that's part of football."
The Gunners welcome Belarusian Premier League champions BATE on Thursday having already sealed top spot in Group H.
Wenger has made mass changes throughout the competition, handing valuable game time to the likes of Theo Walcott, Jack Wilshere and Olivier Giroud.
Even though progression into the last 32 has been assured, the Frenchman still intends to play those on the fringes of the first-team, rather than blood a team of youngsters in what is all-but a dead rubber for the hosts.
Asked how winning the group would affect his team selection, Wenger said: "Not too much, because I have many players available who need competition.
"Next week we go into a week with three games and so some players need competition, some experienced players
We'll play quite an experienced team tomorrow."
Defender Shkodran Mustafi would not have been involved even if he had not been forced off early on in the defeat to United.
Wenger remains hopeful the Germany international could yet return to the side at Southampton on Sunday after suffering a thigh injury at the weekend.
The Arsenal boss also reacted to pictures of Wilshere, a player with a history of injuries in the previous three years, ice skating with his family earlier in the week.
"It's not ideal," he added.
"For your balance it's not bad but for a footballer I don't think it's great
It depends as well on what you do.
"You can do ice skating by bringing your child on and just making sure nothing happens to him, or (you can do) ice skating violently, I don't think he did that."
Following Wenger's press conference, the club announced on Wednesday that youngsters Eddie Nketiah, Ben Sheaf and Matt Macey had penned new deals.
Source : PA
Source: PA