"If I am a player, and I think with what we produced, we have a huge potential, and we lose the game, we can not accept that".
"He had tests on Friday morning and we thought he wouldn't play".
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.
Lacazette surprisingly started and played the full 90 minutes in the 3-1 loss to United on Saturday, scoring Arsenal's goal in the second half, despite Wenger having ruled him out with a groin injury three days earlier.
"I know what I'm capable of doing, I just want to stay fit and healthy and let my football take care of itself", he told the Evening Standard.
Mourinho aimed a dig at Wenger and Manchester City manager Pep Guardiola ahead of this weekend's Manchester derby by repeatedly saying he was telling "the truth" about his team news with "no stories of Lacazette or [David] Silva".
Fellow Frenchman Olivier Giroud has found himself excluded from Premier League action, as has England's Theo Walcott.
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Wenger remains hopeful the Germany global could yet return to the side at Southampton on Sunday after suffering a thigh injury at the weekend. "One of the positives is that the behaviour of the squad is remarkable and impeccable".
Wenger said it was hard to accept the result after playing so well, but believes it should make his players even hungrier.
"I will adapt every time and try to pick a balanced team".
Arsenal have already won their group so Wenger can carry on blooding youngsters like Ainsley Maitland-Niles and Joe Willock.
Wenger has never been a fan of parachuting third place teams into the lesser competition but had rarely had to worry about it.
"Yes, I think he works very hard to come back into it", he said.
"I believe the game, it has to make us angry and even more determined", he said. "It's like the FA Cup, it's important we have a go and give absolutely everything".