The 32-year-old revealed earlier this month he was told the problem might have left him unable to walk.
"It is the worst injury I have known, starting with a little pain in the Achilles and it has gone from worse to worse", Wenger told a news conference.
"I think, based on his ability, he is the best football player I have played with". He has competent medical people around him. He did not play away from home at City, that's the only one, and overall I trust him, ' said Wenger when asked if he did not trust Lacazette to star the biggest games, forgetting he had also axed his record purchase from the side which started against Liverpool back in September.
Wenger confirmed Olivier Giroud will miss Saturday's north London derby at home to Tottenham Hotspur after the France striker injured himself on global duty.
"I'd have Aaron Ramsey in central midfield with Granit Xhaka, but my concern is that Wenger sticks with his principle of playing two holding midfielders, even though he doesn't have the right players to play in those positions".
Lacazette, who scored twice in France's 2-2 friendly draw with Germany, has completed 90 minutes only once since his close-season move from Lyon. Wenger left both Ozil and Sanchez on the bench during their clash at Stamford Bridge.
"I have a big choice to make every time, big decisions to make up front", said Wenger, whose side sit in sixth place in the Premier League table, four points below third-place Spurs.
Wenger, who has been in management for three decades, said he has never seen anything like it.
Cazorla has been posting regular updates on social media with him working out in the gym as he tries to complete his comeback. I think the question about the balance of power in north London is a question we have to answer and the only way to answer is on the pitch.