The France global 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.
The weekend's woes were completed when the league leaders Man City scored a very late victor after it looked like they may be held by West Ham and that goal gives them a huge gap of 15 points over Arsenal.
The United boss said: "Do you want the truth?" "It's the truth, eh?"
"I am not here to especially blame people", Wenger added. "What did you want me to do?"
'The truth is Eric Bailly no chance for the weekend, Phil Jones a chance, [Marouane] Fellaini a chance, Zlatan [Ibrahimovic] a big chance, Matic is injured but will play for sure, I'm telling the truth but he will play for sure, [Michael] Carrick no chance, ' the Portuguese said.
"He had tests on Friday morning and we thought he wouldn't play".
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The plan was when they [Arsenal] had the ball, we all had to defend, starting with the attacking players, and when we have the ball, we had to try to counter-attack fast and try to score goals, which we did.
The Gunners welcome Belarusian Premier League champions BATE on Thursday having already sealed top spot in Group H.
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.
When asked about Mourinho's comments, Wenger said: "I thought he wouldn't play but in the end he could play".
I think even at 2-1 we can not afford to concede another goal.
"That's why it's important in these rounds that we give some young players experience".